[BOLT] Add unit test for SPE PBT feature

When the SPE previous branch target address (named as PBT) feature is
available, an SPE sample by combining this PBT feature, has two entries.
Arm SPE records SRC/DEST addresses of the latest sampled branch operation,
and it stores into the first entry.
PBT records the target address of most recently taken branch in program order
before the sampled operation, it places into the second entry.
They are formed a chain of two consecutive branches.

Where:
- The previous branch operation (PBT) is always taken.
- In SPE entry, the current source branch (SRC) may be either fall-through or taken.
- The target address (DEST) of the recorded branch operation is always
  what was architecturally executed.

However PBT doesn't provide as much information as SPE does. It lacks those information
such as the address of source branch, branch type, and prediction bit.
These information are always filled with zero in PBT entry.
Therefore Bolt cannot evaluate the prediction, and source branch fields,
it leaves them zero during the aggregation process.

Consider the following example to see how SPE profile looks like combining with PBT:

`<PID> <SRC>/<DEST>/PN/-/-/10/COND/- <NULL>/<PBT>/-/-/-/0//-
0xffff8000807216b4/0xffff800080721704/P/-/-/1/COND/-  0x0/0xffff8000807216ac/-/-/-/0//-`

Author: kaadam

bolt/unittests/Profile/PerfSpeEvents.cpp

@@ -161,4 +161,76 @@ TEST_F(PerfSpeEventsTestHelper, SpeBranchesWithBrstack) {
   parseAndCheckBrstackEvents(1234, ExpectedSamples);
 }
 
+TEST_F(PerfSpeEventsTestHelper, SpeBranchesWithBrstackAndPbt) {
+  // Check perf input with SPE branch events as brstack format by
+  // combining with the previous branch target address (named as PBT).
+  // Example collection command:
+  // ```
+  // perf record -e 'arm_spe_0/branch_filter=1/u' -- BINARY
+  // ```
+  // How Bolt extracts the branch events:
+  // ```
+  // perf script -F pid,brstack --itrace=bl
+  // ```
+
+  opts::ArmSPE = true;
+  opts::ReadPerfEvents =
+      // "<PID> <SRC>/<DEST>/PN/-/-/10/COND/- <NULL>/<PBT>/-/-/-/0//-\n"
+      "  4567  0xa002/0xa003/PN/-/-/10/COND/- 0x0/0xa001/-/-/-/0//-\n"
+      "  4567  0xb002/0xb003/P/-/-/4/RET/- 0x0/0xb001/-/-/-/0//-\n"
+      "  4567  0xc456/0xc789/P/-/-/13/-/- 0x0/0xc123/-/-/-/0//-\n"
+      "  4567  0xd456/0xd789/M/-/-/7/RET/- 0x0/0xd123/-/-/-/0//-\n"
+      "  4567  0xe005/0xe009/P/-/-/14/RET/- 0x0/0xe001/-/-/-/0//-\n"
+      "  4567  0xd456/0xd789/M/-/-/7/RET/- 0x0/0xd123/-/-/-/0//-\n"
+      "  4567  0xf002/0xf003/MN/-/-/8/COND/- 0x0/0xf001/-/-/-/0//-\n"
+      "  4567  0xc456/0xc789/P/-/-/13/-/- 0x0/0xc123/-/-/-/0//-\n";
+
+  // ExpectedSamples contains the aggregated information about
+  // a branch {{From, To, TraceTo}, {TakenCount, MispredCount}}.
+  // When the SPE previous branch target address (named as PBT)
+  // feature is available, an SPE sample by combining this PBT feature,
+  // has two entries.
+  // Arm SPE records SRC/DEST addresses of the latest sampled branch operation,
+  // and it stores into the first entry. PBT records the target address of
+  // most recently taken branch in program order before the sampled operation,
+  // it places into the second entry.
+  // They are formed a chain of two consecutive branches.
+  // Where:
+  //   - The previous branch operation (PBT) is always taken.
+  //   - In SPE entry, the current source branch (SRC) may be either
+  //     fall-through or taken.
+  //   - The target address (DEST) of the recorded
+  //     branch operation is always what was architecturally executed.
+  // However PBT lacks associated information such as branch
+  // source address, branch type, and prediction bit.
+  // Considering this Trace pair:
+  //  {{0xd456, 0xd789, Trace::BR_ONLY}, {2, 2}},
+  //    {{0x0, 0xd123, 0xd456}, {2, 0}}
+  // For SPE trace please see the description above.
+  // The second entry is the PBT trace:
+  // {{0x0, 0xd123, 0xd456}, {2, 0}}.
+  // The PBT entry has a TakenCount = 2, as we have two samples for
+  // (0x0, 0xd123) entry in our input. The 'MispredsCount = 0' is
+  // always zero, because it lacks prediction information.
+  // It also has no information about source branch address therefore
+  // Bolt doesn't evaluate the 'From' field, and leaves it as zero (0x0).
+  // TraceTo = 0xc456, means the execution jumped from
+  // 0xc123 (PBT) to 0xc456 (SRC), and jumped further to 0xd789 (DEST).
+  std::vector<std::pair<Trace, TakenBranchInfo>> ExpectedSamples = {
+      {{0xa002, 0xa003, Trace::BR_ONLY}, {1, 0}},
+      {{0x0, 0xa001, 0xa002}, {1, 0}},
+      {{0xb002, 0xb003, Trace::BR_ONLY}, {1, 0}},
+      {{0x0, 0xb001, 0xb002}, {1, 0}},
+      {{0xc456, 0xc789, Trace::BR_ONLY}, {2, 0}},
+      {{0x0, 0xc123, 0xc456}, {2, 0}},
+      {{0xd456, 0xd789, Trace::BR_ONLY}, {2, 2}},
+      {{0x0, 0xd123, 0xd456}, {2, 0}},
+      {{0xe005, 0xe009, Trace::BR_ONLY}, {1, 0}},
+      {{0x0, 0xe001, 0xe005}, {1, 0}},
+      {{0xf002, 0xf003, Trace::BR_ONLY}, {1, 1}},
+      {{0x0, 0xf001, 0xf002}, {1, 0}}};
+
+  parseAndCheckBrstackEvents(4567, ExpectedSamples);
+}
+
 #endif