[X86][SSE] Avoid scalarization of v2i64 vector shifts

Currently v2i64 vectors shifts (non-equal shift amounts) are scalarized, costing 4 x extract, 2 x x86-shifts and 2 x insert instructions - and it gets even more awkward on 32-bit targets.

This patch separately shifts the vector by both shift amounts and then shuffles the partial results back together, costing 2 x shuffles and 2 x sse-shifts instructions (+ 2 movs on pre-AVX hardware).

Note - this patch only improves the SHL / LSHR logical shifts as only these are supported in SSE hardware.

Differential Revision: http://reviews.llvm.org/D8416

llvm-svn: 232660