[llvm] [X86][MC] Support encoding of EGPR for APX (PR #71909)

[Shengchen Kan via llvm-commits]

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@@ -121,11 +126,14 @@ class X86OpcodePrefixHelper {
   unsigned R : 1;
   unsigned X : 1;
   unsigned B : 1;
+  unsigned M : 1;
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KanRobert wrote:

Yes. `M` bit is used encode MAP 0/1 in REX2. In EVEX, we use `mmmm`

https://github.com/llvm/llvm-project/pull/71909