[X86][SSE] Add pre-AVX2 support for (i32 bitcast(v32i1)) -> 2xMOVMSK

Currently we only support (i32 bitcast(v32i1)) using the AVX2 VPMOVMSKB ymm instruction.

This patch adds support for splitting pre-AVX2 targets into 2 x (V)PMOVMSKB xmm instructions and merging the integer results.

In future we could probably generalize this to handle more cases.

Differential Revision: https://reviews.llvm.org/D35303

llvm-svn: 308723