[CFGMST] Remove special case for entry block with no successors

This breaks in the case where there are unreachable blocks after an entry block with no successors, which don't get visited in CFGMST, causing crashes.

Fixes #135828.

Author: aeubanks

llvm/include/llvm/Transforms/Instrumentation/CFGMST.h

@@ -133,12 +133,6 @@ template <class Edge, class BBInfo> class CFGMST {
     LLVM_DEBUG(dbgs() << "  Edge: from fake node to " << Entry->getName()
                       << " w = " << EntryWeight << "\n");
 
-    // Special handling for single BB functions.
-    if (succ_empty(Entry)) {
-      addEdge(Entry, nullptr, EntryWeight);
-      return;
-    }
-
     static const uint32_t CriticalEdgeMultiplier = 1000;
 
     for (BasicBlock &BB : F) {

llvm/test/Transforms/PGOProfile/Inputs/unreachable-block.proftext

@@ -0,0 +1,9 @@
+# IR level Instrumentation Flag
+:ir
+foo
+# Func Hash:
+742261418966908927
+# Num Counters:
+1
+# Counter Values:
+1

llvm/test/Transforms/PGOProfile/unreachable-block.ll

@@ -0,0 +1,12 @@
+; RUN: llvm-profdata merge %S/Inputs/unreachable-block.proftext -o %t.profdata
+; RUN: opt < %s -passes=pgo-instr-use -pgo-test-profile-file=%t.profdata -S
+
+declare ptr @bar()
+
+define ptr @foo() {
+entry:
+  ret ptr null
+
+2:
+  ret ptr null
+}