[llvm] [X86] Support EVEX compression for EGPR (PR #77202)

[Shengchen Kan via llvm-commits]

================
@@ -29,8 +29,10 @@ class X86Subtarget;
 namespace X86 {
 
 enum AsmComments {
+  // For instr that was compressed from EVEX to LEGACY.
+  AC_EVEX_2_LEGACY = MachineInstr::TAsmComments,
----------------
KanRobert wrote:

```
  enum CommentFlag {
    ReloadReuse = 0x1,    // higher bits are reserved for target dep comments.
    NoSchedComment = 0x2,
    TAsmComments = 0x4    // Target Asm comments should start from this value.
  };
```

It's documented at llvm/include/llvm/CodeGen/MachineInstr.h. `T` is short for target and `TAsmComments` should always be the last value here.


https://github.com/llvm/llvm-project/pull/77202