[x86] Clean up the shift lowering vector shuffle tests a bit using my
script. Notably this folds all the SSE cases together into a single
FileCheck block. It also adds a vex prefix.
[x86] Cleanup the combining vector shuffle tests a bit by merging
identical checks for different SSE variants into a single block.
[x86] Switch a constant selection test to use positive assertions and to
store to real pointers so that its clear that the right code is in fact
being generated.
[x86] Clean up the SSE1 test to use a slightly different pattern for
matching offsets. I don't expect this to really matter, but its what the
latest incarnation of my script for maintaining these tests happens to
produce, and so its simpler for me if everything matches.