[llvm] [X86] Fix incorrect NOP insertion between fused instructions that breaks macro fusion (PR #155316)

[Aleksandr Popov via llvm-commits]

================
@@ -0,0 +1,14 @@
+# RUN: llvm-mc -filetype=obj -triple x86_64 --x86-align-branch-boundary=32 --x86-align-branch=fused+jcc %s | llvm-objdump -d --no-show-raw-insn - | FileCheck %s
+
+# Exercise the case where fused instructions need to be aligned,
+# ensuring fusion is not broken by a NOP
+
+        .globl  f
+f:
+        .nops 27
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aleks-tmb wrote:

I've restored the `-x86-pad-max-prefix-size=1` option to make the issue reproducible without the patch. Without this option, we do not emit `test` instruction into a fragment, due to the check below:
```
  if (!(Backend.mayNeedRelaxation(Inst.getOpcode(), Inst.getOperands(), STI) ||
        Backend.allowEnhancedRelaxation())) {
    emitInstToData(Inst, STI);
    return;
  }
```

https://github.com/llvm/llvm-project/pull/155316