[PGO] Skip if an IndirectBrInst critical edge cannot be split

PGOInstrumentation runs `SplitIndirectBrCriticalEdges` but some IndirectBrInst
critical edge cannot be split. `getInstrBB` will crash when calling `SplitCriticalEdge`, e.g.

  int foo(char *p) {
    void *targets[2];
    targets[0] = &&indirect;
    targets[1] = &&end;
    for (;; p++)
      if (*p == 7) {
  indirect:
        goto *targets[p[1]]; // the self loop is critical in -O
      }
  end:
    return 0;
  }

Skip such critical edges to prevent a crash.

Reviewed By: davidxl, lebedev.ri

Differential Revision: https://reviews.llvm.org/D87435