[x86] Remove a test that provides little value. There are plenty of
tests for zext of a vector.
[x86] Remove a couple of other overly isolated tests that are low-value
at this point. We have lots of tests of peephole optimizations with
insert and extract on vectors.
[x86] Regenerate a number of FileCheck assertions with my script for
test cases that will change with the new vector shuffle lowering. This
gives us a nice baseline for deltas against. I've checked and removed
the cases where there were weird register usage being pinned down, and
all of these are extremely pin-pointed tests so fully checking them
seems very appropriate.