[AMDGPU][AsmParser] Refine parsing instruction operands.

Eliminates the need for working around optional and token operands being
mistakenly parsed as expressions.

Reviewed By: dp

Differential Revision: https://reviews.llvm.org/D138492

Author: kosarev

llvm/include/llvm/Target/Target.td

@@ -1479,6 +1479,16 @@ class AsmParser {
   // to matching the parsed instruction, so to allow more detailed error
   // messages.
   bit ReportMultipleNearMisses = false;
+
+  // OperandParserMethod - If non-empty, this is the name of a custom
+  // member function of the AsmParser class to call for every instruction
+  // operand to be parsed.
+  string OperandParserMethod = "";
+
+  // CallCustomParserForAllOperands - Set to true if the custom parser
+  // method shall be called for all operands as opposed to only those
+  // that have their own specified custom parsers.
+  bit CallCustomParserForAllOperands = false;
 }
 def DefaultAsmParser : AsmParser;
 

llvm/lib/Target/AMDGPU/AMDGPU.td

@@ -1337,6 +1337,10 @@ def AMDGPUAsmParser : AsmParser {
   // Some of the R600 registers have the same name, so this crashes.
   // For example T0_XYZW and T0_XY both have the asm name T0.
   let ShouldEmitMatchRegisterName = 0;
+
+  // Call the custom operand parser for all operands.
+  let OperandParserMethod = "parseCustomOperand";
+  let CallCustomParserForAllOperands = true;
 }
 
 def AMDGPUAsmWriter : AsmWriter {