ARM: Use MCTargetAsmParser::validateTargetOperandClass().

Use the validateTargetOperandClass() hook to match literal '#0' operands in
InstAlias definitions. Previously this required per-instruction C++ munging of the
operand list, but not is handled as a natural part of the matcher. Much better.

No additional tests are required, as the pre-existing tests for these instructions
exercise the new behaviour as being functionally equivalent to the old.

llvm-svn: 174488