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

#70958 adds registers R16-R31 (EGPR), this patch
1. Introduces a new instruction prefix REX2
2. Supports encoding of EGPR with REX2 for legacy instructions in MAP
0/1
3. Supports encoding of EGPR with EVEX for the existing instructions in
EVEX space

RFC:

https://discourse.llvm.org/t/rfc-design-for-apx-feature-egpr-and-ndd-support/73031/4

llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp

@@ -106,6 +106,9 @@ class X86AsmParser : public MCTargetAsmParser {
 
   DispEncoding ForcedDispEncoding = DispEncoding_Default;
 
+  // Does this instruction use apx extended register?
+  bool UseApxExtendedReg = false;
+
 private:
   SMLoc consumeToken() {
     MCAsmParser &Parser = getParser();
@@ -1410,6 +1413,9 @@ bool X86AsmParser::MatchRegisterByName(MCRegister &RegNo, StringRef RegName,
     }
   }
 
+  if (X86II::isApxExtendedReg(RegNo))
+    UseApxExtendedReg = true;
+
   // If this is "db[0-15]", match it as an alias
   // for dr[0-15].
   if (RegNo == 0 && RegName.startswith("db")) {
@@ -3084,6 +3090,7 @@ bool X86AsmParser::ParseInstruction(ParseInstructionInfo &Info, StringRef Name,
   // Reset the forced VEX encoding.
   ForcedVEXEncoding = VEXEncoding_Default;
   ForcedDispEncoding = DispEncoding_Default;
+  UseApxExtendedReg = false;
 
   // Parse pseudo prefixes.
   while (true) {
@@ -3954,6 +3961,9 @@ unsigned X86AsmParser::checkTargetMatchPredicate(MCInst &Inst) {
   unsigned Opc = Inst.getOpcode();
   const MCInstrDesc &MCID = MII.get(Opc);
 
+  if (UseApxExtendedReg && !X86II::canUseApxExtendedReg(MCID))
+    return Match_Unsupported;
+
   if (ForcedVEXEncoding == VEXEncoding_EVEX &&
       (MCID.TSFlags & X86II::EncodingMask) != X86II::EVEX)
     return Match_Unsupported;

llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h

@@ -1208,6 +1208,12 @@ namespace X86II {
     return RegNo >= X86::ZMM0 && RegNo <= X86::ZMM31;
   }
 
+  /// \returns true if \p RegNo is an apx extended register.
+  inline bool isApxExtendedReg(unsigned RegNo) {
+    assert(X86::R31WH - X86::R16 == 95 && "EGPRs are not continuous");
+    return RegNo >= X86::R16 && RegNo <= X86::R31WH;
+  }
+
   /// \returns true if the MachineOperand is a x86-64 extended (r8 or
   /// higher) register,  e.g. r8, xmm8, xmm13, etc.
   inline bool isX86_64ExtendedReg(unsigned RegNo) {
@@ -1218,6 +1224,9 @@ namespace X86II {
         (RegNo >= X86::ZMM8 && RegNo <= X86::ZMM31))
       return true;
 
+    if (isApxExtendedReg(RegNo))
+      return true;
+
     switch (RegNo) {
     default: break;
     case X86::R8:    case X86::R9:    case X86::R10:   case X86::R11: