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.

There is still another optimization possible by using a cmov instruction, provided it's available on the CPU that we are compiling for, but I wanted to make sure I'm going about this the right way before I continue.