[SystemZ][z/OS] Enforce prefix-less registers in SystemZAsmParser for the HLASM dialect.

- Previously, https://reviews.llvm.org/D101308 removed prefixes from register while printing them out. This was especially needed for inline asm statements which used input/output operands.
- However, the backend SystemZAsmParser, accepts both prefixed registers and prefix-less registers as part of its implementation
- This patch aims to change that by ensuring that prefixed registers are only allowed for the ATT dialect.

Reviewed By: uweigand

Differential Revision: https://reviews.llvm.org/D101665

Author: aniprasad

llvm/lib/Target/SystemZ/AsmParser/SystemZAsmParser.cpp

@@ -830,7 +830,7 @@ SystemZAsmParser::parseRegister(OperandVector &Operands, RegisterKind Kind) {
   }
 
   // Handle register names of the form %<prefix><number>
-  if (Parser.getTok().is(AsmToken::Percent)) {
+  if (isParsingATT() && Parser.getTok().is(AsmToken::Percent)) {
     if (parseRegister(Reg))
       return MatchOperand_ParseFail;
 
@@ -912,6 +912,9 @@ SystemZAsmParser::parseAnyRegister(OperandVector &Operands) {
     Operands.push_back(SystemZOperand::createImm(Register, StartLoc, EndLoc));
   }
   else {
+    if (isParsingHLASM())
+      return MatchOperand_NoMatch;
+
     Register Reg;
     if (parseRegister(Reg))
       return MatchOperand_ParseFail;
@@ -1019,7 +1022,7 @@ bool SystemZAsmParser::parseAddress(bool &HaveReg1, Register &Reg1,
   if (getLexer().is(AsmToken::LParen)) {
     Parser.Lex();
 
-    if (getLexer().is(AsmToken::Percent)) {
+    if (isParsingATT() && getLexer().is(AsmToken::Percent)) {
       // Parse the first register.
       HaveReg1 = true;
       if (parseRegister(Reg1))
@@ -1062,7 +1065,7 @@ bool SystemZAsmParser::parseAddress(bool &HaveReg1, Register &Reg1,
         if (parseIntegerRegister(Reg2, RegGR))
           return true;
       } else {
-        if (parseRegister(Reg2))
+        if (isParsingATT() && parseRegister(Reg2))
           return true;
       }
     }
@@ -1419,7 +1422,7 @@ bool SystemZAsmParser::parseOperand(OperandVector &Operands,
   // a context-dependent parse routine, which gives the required register
   // class.  The code is here to mop up other cases, like those where
   // the instruction isn't recognized.
-  if (Parser.getTok().is(AsmToken::Percent)) {
+  if (isParsingATT() && Parser.getTok().is(AsmToken::Percent)) {
     Register Reg;
     if (parseRegister(Reg))
       return true;