[InstCombine] recognize bswap disguised as shufflevector


[InstCombine] recognize bswap disguised as shufflevector

bitcast <N x i8> (shuf X, undef, <N, N-1,...0>) to i{N*8} --> bswap (bitcast X to i{N*8})

In PR43146:
...we have a more complicated case where SLP is making a mess of bswap. This patch won't
do anything for that currently, but we need to improve bswap recognition in instcombine,
SLP, and/or a standalone pass to avoid that problem.

This is limited using the data-layout so we don't try to do this transform with actual
vector types. The backend does not appear to have folds to convert in either direction,
so we don't want to mess up something that is actually better lowered as a shuffle.

On x86, we're trading something like this:

vmovd	%edi, %xmm0
vpshufb	LCPI0_0(%rip), %xmm0, %xmm0 ## xmm0 = xmm0[3,2,1,0,u,u,u,u,u,u,u,u,u,u,u,u]
vmovd	%xmm0, %eax


movl	%edi, %eax
bswapl	%eax

Differential Revision: https://reviews.llvm.org/D66965