Allow targets to add custom asm operand matching logic.

Allow targets to add custom asm operand matching logic.

For example, ARM has several instructions with a literal '#0' immediate in the syntax
that's not represented as an actual operand. The asm matcher is expected a token
operand, but the parser will have created an immediate operand. This is currently
handled by dedicated per-instruction C++ munging of the ParsedAsmOperand list, but
will be better handled by this hook.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174487 91177308-0d34-0410-b5e6-96231b3b80d8