[X86] Lowering integer truncation intrinsics to native IR

This patch lowers the _mm[256|512]_cvtepi{64|32|16}_epi{32|16|8} intrinsics to
native IR in cases where the result's length is less than 128 bits.

The resulting IR for 256-bit inputs is folded into VPMOV instructions, while for
128-bit inputs the vpshufb (or, in the 64-to-32-bit case, vinsertps)
instructions are generated instead

Differential Revision: https://reviews.llvm.org/D48712

llvm-svn: 336643