Implement support for custom target specific asm parsing of operands.

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