[x86] Re-work a bunch of the v32i8 test cases to actually involve byte
shuffles rather than word shuffles.

As you might guess, these were built starting from the word shuffle test
cases and I failed to properly port a bunch of them and left them as
widened word shuffle test cases. We still have a couple of tests that
check our ability to widen shuffles, but now we will test the actual
byte shuffle quite a bit better.