Implement IRGen for the x86 vectorcall convention

The most complex aspect of the convention is the handling of homogeneous
vector and floating point aggregates.  Reuse the homogeneous aggregate
classification code that we use on PPC64 and ARM for this.

This convention also has a C mangling, and we apparently implement that
in both Clang and LLVM.

Reviewed By: majnemer

Differential Revision: http://reviews.llvm.org/D6063

llvm-svn: 221006