[x86] Refactor all of the VSELECT-as-blend lowering code to avoid domain
crossing and generally work more like the blend emission code in the new
vector shuffle lowering.

My goal is to have the new vector shuffle lowering just produce VSELECT
nodes that are either matched here to BLENDI or are legal and matched in
the .td files to specific blend instructions. That seems much cleaner as
there are other ways to produce a VSELECT anyways. =]

No *observable* functionality changed yet, mostly because this code
appears to be near-dead. The behavior of this lowering routine did
change though. This code being mostly dead and untestable will change
with my next commit which will also point some new tests at it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218588 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Delete a bunch of really bad and totally unnecessary code in the
X86 target-specific DAG combining that tried to convert VSELECT nodes
into VECTOR_SHUFFLE nodes that it "knew" would lower into
immediate-controlled blend nodes.

Turns out, we have perfectly good lowering of all these VSELECT nodes,
and indeed that lowering already knows how to handle lowering through
BLENDI to immediate-controlled blend nodes. The code just wasn't getting
used much because this thing forced the world to go through the vector
shuffle lowering. Yuck.

This also exposes that I was too aggressive in avoiding domain crossing
in v218588 with that lowering -- when the other option is to expand into
two 128-bit vectors, it is worth domain crossing. Restore that behavior
now that we have nice tests covering it.

The test updates here fall into two camps. One is where previously we
ended up with an unsigned encoding of the blend operand and now we get
a signed encoding. In most of those places there were elaborate comments
explaining exactly what these operands really mean. Rather than that,
just switch these tests to use the nicely decoded comments that make it
obvious that the final shuffle matches.

The other updates are just removing pointless domain crossing by
blending integers with PBLENDW rather than BLENDPS.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218589 91177308-0d34-0410-b5e6-96231b3b80d8