Initial fastisel call support for C, Fast, and X86_FastCall calling...

Initial fastisel call support for C, Fast, and X86_FastCall calling conventions. It's meant to handle "simple" calls, i.e. no byval, structret, etc. It doesn't support multi-result returns either.

Not yet turned on, it needs to support sext / zext of arguments and result.

llvm-svn: 55882