[llvm] [X86][MC] Support encoding/decoding for JMPABS (PR #72835)

[Shengchen Kan via llvm-commits]

================
@@ -155,6 +155,8 @@ namespace X86Local {
   enum {
     AdSize16 = 1, AdSize32 = 2, AdSize64 = 3
   };
+
+  enum { ExplicitREX2 = 1 };
----------------
KanRobert wrote:

If no this enum, then in X86RecognizableInstr.cpp, the code would be
```
  ExplicitREX2Prefix =
      byteFromRec(Rec, "explicitOpPrefixBits") == 1;
```
which was hard to read.

https://github.com/llvm/llvm-project/pull/72835