[x86] Teach the x86 DAG combiner to form UNPCKLPS and UNPCKHPS
instructions from the relevant shuffle patterns.

This is the last tweak I'm aware of to generate essentially perfect
v4f32 and v2f64 shuffles with the new vector shuffle lowering up through
SSE4.1. I'm sure I've missed some and it'd be nice to check since v4f32
is amenable to exhaustive exploration, but this is all of the tricks I'm
aware of.

With AVX there is a new trick to use the VPERMILPS instruction, that's
coming up in a subsequent patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217761 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Add an explicit SSE3 run to this test and flesh out a bunch of
missing specific checks.

While there is a lot of redundancy here where all-but-one mode use the
same code generation, I'd rather have each variant spelled out and
checked so that readers aren't misled by an omission in the test suite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217765 91177308-0d34-0410-b5e6-96231b3b80d8