[X86] Failure to merge BT with matching BTC/BTR/BTS

[RKSimon]

define i1 @btr_eq_i32(ptr %word, i32 %position) nounwind {
  %ofs = and i32 %position, 31
  %bit = shl nuw i32 1, %ofs
  %mask = xor i32 %bit, -1
  %ld = load i32, ptr %word
  %test = and i32 %ld, %bit
  %res = and i32 %ld, %mask
  %cmp = icmp eq i32 %test, 0
  store i32 %res, ptr %word
  ret i1 %cmp
}

btr_eq_i32: # @btr_eq_i32
  movl (%rdi), %eax
  movl %eax, %ecx
  btrl %esi, %ecx
  btl %esi, %eax
  setae %al
  movl %ecx, (%rdi)
  retq

The BTR (or BTC/BTS) sets the same EFLAGS CF result as BT (it then clears the bit value afterward) - so we could perform this as:

btr_eq_i32: # @btr_eq_i32
  movl (%rdi), %eax
  btrl %esi, %eax
  movl %eax, (%rdi)
  setae %al
  retq

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

```ll define i1 @btr_eq_i32(ptr %word, i32 %position) nounwind { %ofs = and i32 %position, 31 %bit = shl nuw i32 1, %ofs %mask = xor i32 %bit, -1 %ld = load i32, ptr %word %test = and i32 %ld, %bit %res = and i32 %ld, %mask %cmp = icmp eq i32 %test, 0 store i32 %res, ptr %word ret i1 %cmp } ``` ```asm btr_eq_i32: # @btr_eq_i32 movl (%rdi), %eax movl %eax, %ecx btrl %esi, %ecx btl %esi, %eax setae %al movl %ecx, (%rdi) retq ``` The BTR sets the same EFLAGS CF result as BT (it then clear the bit value afterward) - so we could perform this as: ```asm btr_eq_i32: # @btr_eq_i32 movl (%rdi), %eax btrl %esi, %eax movl %eax, (%rdi) setae %al retq ```

[LoNgLoNgDoUblE]

Hey @RKSimon I'd like to work on this.