[llvm] [BOLT] pacret-scanner: fix regression test failure (PR #128576)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

> My git bisect run ended up pointing to commit [52fc6ff](https://github.com/llvm/llvm-project/commit/52fc6ffcda0895c0c7b976ad1f5cb5a282b571d2). I'd need to find some time to understand why this commit is causing the behavioural change in `llvm-bolt-binary-analysis`...

It seems only calls to unresolved symbols are affected, in that a new basic block is started after a call to an unresolved symbol, but not in a call to a resolved symbol. I'm guessing this doesn't happen (often) in real binaries, but does happen frequently in our regression tests.

A small reproducer showing this is:
```sh
#!/bin/bash
CLANG=./bin/clang
BOLT=./bin/llvm-bolt
cat << EOF > ./test_bolt_bb.s
        .text
        .globl  f1
        .type   f1, at function
f1:
        stp     x29, x30, [sp, #-16]!
        mov     x29, sp
        bl      foo       // unresolved symbol
        ldp     x29, x30, [sp], #16
        ret
        .size f1, .-f1


        .globl  f2
        .type   f2, at function
f2:
        stp     x29, x30, [sp, #-16]!
        mov     x29, sp
        bl      f1      // resolved symbol
        ldp     x29, x30, [sp], #16
        ret
        .size f2, .-f2
EOF
$CLANG -fuse-ld=lld -Wl,--unresolved-symbols=ignore-all --target=aarch64-linux-gnu -nostartfiles -nostdlib -ffreestanding ./test_bolt_bb.s -o ./test_bolt_bb.exe
$BOLT -o tmp.exe --print-cfg --no-threads  ./test_bolt_bb.exe
```

produces:
```
...
Binary Function "f1" after building cfg {
  Number      : 1
  State       : CFG constructed
  Address     : 0x10258
  Size        : 0x14
  MaxSize     : 0x14
  Offset      : 0x258
  Section     : .text
  Orc Section : .local.text.f1
  LSDA        : 0x0
  IsSimple    : 1
  IsMultiEntry: 0
  IsSplit     : 0
  BB Count    : 2
  Hash        : df55f35eb8a34774
  BB Layout   : .LBB00, .Ltmp0
}
.LBB00 (3 instructions, align : 1)
  Entry Point
    00000000:   stp     x29, x30, [sp, #-0x10]!
    00000004:   mov     x29, sp
    00000008:   bl      .Ltmp0
  Successors: .Ltmp0

.Ltmp0 (2 instructions, align : 1)
  Predecessors: .LBB00
    0000000c:   ldp     x29, x30, [sp], #0x10
    00000010:   ret

DWARF CFI Instructions:
    <empty>
End of Function "f1"

Binary Function "f2" after building cfg {
  Number      : 2
  State       : CFG constructed
  Address     : 0x1026c
  Size        : 0x14
  MaxSize     : 0x14
  Offset      : 0x26c
  Section     : .text
  Orc Section : .local.text.f2
  LSDA        : 0x0
  IsSimple    : 1
  IsMultiEntry: 0
  IsSplit     : 0
  BB Count    : 1
  Hash        : df55f35eb8a34774
  BB Layout   : .LBB01
}
.LBB01 (5 instructions, align : 1)
  Entry Point
    00000000:   stp     x29, x30, [sp, #-0x10]!
    00000004:   mov     x29, sp
    00000008:   bl      f1
    0000000c:   ldp     x29, x30, [sp], #0x10
    00000010:   ret

...
```

https://github.com/llvm/llvm-project/pull/128576