Implement support for custom target specific asm parsing of operands.

Motivation: Improve the parsing of not usual (different from registers or
immediates) operand forms.

This commit implements only the generic support. The ARM specific modifications
will come next.

A table like the one below is autogenerated for every instruction
containing a 'ParserMethod' in its AsmOperandClass

static const OperandMatchEntry OperandMatchTable[20] = {
 /* Mnemonic, Operand List Mask, Operand Class, Features */
 { "cdp", 29 /* 0, 2, 3, 4 */, MCK_Coproc, Feature_IsThumb|Feature_HasV6 },
 { "cdp", 58 /* 1, 3, 4, 5 */, MCK_Coproc, Feature_IsARM },

A matcher function very similar (but lot more naive) to
MatchInstructionImpl scans the table. After the mnemonic match, the
features are checked and if the "to be parsed" operand index is
present in the mask, there's a real match. Then, a switch like the one
below dispatch the parsing to the custom method provided in
'ParseMethod':

 case MCK_Coproc:
   return TryParseCoprocessorOperandName(Operands);




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125030 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/Target.td

@@ -341,6 +341,12 @@ class AsmOperandClass {
   /// signature should be:
   ///   void addFooOperands(MCInst &Inst, unsigned N) const;
   string RenderMethod = ?;
+
+  /// The name of the method on the target specific operand to call to custom
+  /// handle the operand parsing. This is useful when the operands do not relate
+  /// to immediates or registers and are very instruction specific (as flags to
+  /// set in a processor register, coprocessor number, ...).
+  string ParserMethod = ?;
 }
 
 def ImmAsmOperand : AsmOperandClass {