[X86] When using vpopcntdq for ctpop of v8i16 vectors, only promote to v8i32.

Previously we promoted to v8i64, but we don't need to go all the way to 512-bits. If we have VLX we can use the 256-bit instruction. And even if we don't have VLX we can widen v8i32 to v16i32 and drop the upper half.

llvm-svn: 320926