[x86] Significantly improve the ability of the new vector shuffle
lowering to match VZEXT_MOVL patterns.

I hadn't realized that these had sufficient pattern smarts in the
backend to lower zext-ing from the low element of a vector without it
being a scalar_to_vector node. They do, and this is how to match a bunch
of patterns for movq, movss, etc.

There is a weird propensity to end up using pshufd to place the element
afterward even though it means domain crossing (or rather, to use
xorps+movss to zext the element rather than movq) but that's an
orthogonal problem with VZEXT_MOVL that someone should probably look at.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218977 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Fix the RUN-lines of this test to make sense.

I got them quite wrong when updating it and had the SSE4.1 run checked
for SSE2 and the SSE2 run checked for SSE4.1. I think everything was
actually generic SSE, but this still seems good to fix. While here,
hoist the triple into the IR and make the flag set a bit more direct in
what it is trying to test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218978 91177308-0d34-0410-b5e6-96231b3b80d8
Fix typo in comment

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218979 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Refactor the element insertion logic in the new vector shuffle
lowering to handle the potential mirroring of 2-element vectors (because
we can't reliably sort them one way) in the caller rather than in the
insertion logic.

This will simplify things considerably as more ways to fail to match the
insertion are added because now we have a nice try and retry point.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218980 91177308-0d34-0410-b5e6-96231b3b80d8