Improved the codegen of the memcmp intrinsic

This modifies the codegen of the memcmp intrinsic so that it will now generate cmpsw, cmpsd, and cmpsq instructions, it also omits the rep and rep count instructions if only 1 instruction was needed. This is the first time I'm poking dmd's backend, so there are probably a few things I don't know, be it coding style, or just how to do certain things, so any feedback is appreciated.