[mips] [IAS] Add support for the XOR $reg,imm pseudo-instruction.

Summary:
This adds a MipsInstAlias which expands to XORi $reg,$reg,imm. For example, "xor $6, 0x3A" should be expanded to "xori $6, $6, 58".
This should work for all MIPS ISAs.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8284

llvm-svn: 232473