Because of the EMMS problem, right now we have to support

user-defined operations that use MMX register types, but
the compiler shouldn't generate them on its own.  This adds
a Synthesizable abstraction to represent this, and changes
the vector widening computation so it won't produce MMX types.
(The motivation is to remove noise from the ABI compatibility
part of the gcc test suite, which has some breakage right now.)

llvm-svn: 101951