[BPF] Don't insert unreachable 'goto' after JX instructions

Update BPFInstrInfo::analyzeBranch() to comply with
TargetInstrInfo::analyzeBranch() requirements for JX instruction:
if branch instruction can't be categorized as a conditional with
true/false branches -- return true.

Because of this bug MachineBlockPlacement transformation inserted an
additional unreachabe jump after JX, e.g.:

  bb.1.entry:
    ...
    JX killed $r1, %jump-table.0
    JMP %bb.2

Additionally, isNotDuplicable annotation is necessary to avoid machine
level transformations creating several JX instruction copies.
Such copies would refer to the same jump table and would make it not
possible to calculate jump offsets inside the table.
Files triggering such duplication are present in kernel selftests.

Author: eddyz87

llvm/lib/Target/BPF/BPFInstrInfo.cpp

@@ -181,9 +181,10 @@ bool BPFInstrInfo::analyzeBranch(MachineBasicBlock &MBB,
     if (!isUnpredicatedTerminator(*I))
       break;
 
-    // If a JX insn, we're done.
-    if (I->getOpcode() == BPF::JX)
-      break;
+    // From base method doc: ... returning true if it cannot be understood ...
+    // Indirect branch has multiple destinations and no true/false concepts.
+    if (I->isIndirectBranch())
+      return true;
 
     // A terminator that isn't a branch can't easily be handled
     // by this analysis.

llvm/lib/Target/BPF/BPFInstrInfo.td

@@ -308,7 +308,7 @@ defm JSLE : J<BPF_JSLE, "s<=", BPF_CC_LE, BPF_CC_LE_32>;
 defm JSET : J<BPF_JSET, "&", NoCond, NoCond>;
 def JCOND : JMP_JCOND<BPF_JCOND, "may_goto", []>;
 
-let isIndirectBranch = 1 in {
+let isIndirectBranch = 1, isBarrier = 1, isNotDuplicable = 1 in {
   def JX : JMP_IND<BPF_JA, "gotox", [(BPFBrJt tjumptable:$jt, i64:$dst)]>;
 }
 }