[X86] Use __builtin_ia32_vec_ext_v4hi and __builtin_ia32_vec_set_v4hi to...

[X86] Use __builtin_ia32_vec_ext_v4hi and __builtin_ia32_vec_set_v4hi to implement pextrw/pinsertw MMX intrinsics instead of trying to use native IR.

Without this we end up generating code that doesn't use mmx registers and probably doesn't work well with other mmx intrinsics.

llvm-svn: 274968