Previously, LLVM was generating a ton of byte-by-byte copies,
leading to huge numbers of vregs and bloating the code. Now, using
memset, the code becomes a nice series of SSE moves instead.

This is needed for a lot of Apple libraries, as Apple tends to put a lot of
globals in dynamic libraries.

Rework method lookup to properly handle self types for non impl matches. Closes #3268. Closes #3274.

This results in a lot of warnings in rustc.  I left them in because
many are bugs and we should fix our code, but Graydon asked that
I not touch every file in the codebase.

Doesn't exactly eliminate a match check, but simplifies the logic a bit

Closes #3245

A similar approach could be used for type parameters.

Fixes #2282.

This eliminates some match checks. Also get rid of other match checks
in metadata code.