[llvm] [AArch64] Improve lowering of scalar abs(sub(a, b)). (PR #151180)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-x86_64-debian-dylib` running on `gribozavr4` while building `llvm` at step 7 "test-build-unified-tree-check-llvm".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/60/builds/34897

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (test-build-unified-tree-check-llvm) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/AArch64/midpoint-int.ll' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
/b/1/llvm-x86_64-debian-dylib/build/bin/llc -mtriple=aarch64-unknown-linux-gnu < /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/midpoint-int.ll | /b/1/llvm-x86_64-debian-dylib/build/bin/FileCheck /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/midpoint-int.ll # RUN: at line 2
+ /b/1/llvm-x86_64-debian-dylib/build/bin/llc -mtriple=aarch64-unknown-linux-gnu
+ /b/1/llvm-x86_64-debian-dylib/build/bin/FileCheck /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/midpoint-int.ll
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/midpoint-int.ll:304:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: sxth w9, w1
              ^
<stdin>:197:10: note: scanning from here
// %bb.0:
         ^
<stdin>:200:7: note: possible intended match here
 subs w10, w9, w1, sxth
      ^
/b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/midpoint-int.ll:429:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: sxtb w9, w1
              ^
<stdin>:279:10: note: scanning from here
// %bb.0:
         ^
<stdin>:282:7: note: possible intended match here
 subs w10, w9, w1, sxtb
      ^

Input file: <stdin>
Check file: /b/1/llvm-x86_64-debian-dylib/llvm-project/llvm/test/CodeGen/AArch64/midpoint-int.ll

-dump-input=help explains the following input dump.

Input was:
<<<<<<
            .
            .
            .
          192:  // -- End function 
          193:  .globl scalar_i16_signed_mem_reg // -- Begin function scalar_i16_signed_mem_reg 
          194:  .p2align 2 
          195:  .type scalar_i16_signed_mem_reg, at function 
          196: scalar_i16_signed_mem_reg: // @scalar_i16_signed_mem_reg 
          197: // %bb.0: 
next:304'0              X error: no match found
          198:  ldrsh w9, [x0] 
next:304'0     ~~~~~~~~~~~~~~~~
          199:  mov w8, #-1 // =0xffffffff 
next:304'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          200:  subs w10, w9, w1, sxth 
next:304'0     ~~~~~~~~~~~~~~~~~~~~~~~~
...

```

</details>

https://github.com/llvm/llvm-project/pull/151180