[x86] Add a 32-bit run to the sext test, and remove a sad vec_sext.ll
test file.

This old test had a bunch of functions that were never even checked. =/
The only thing it really did was to make sure that we did something
reasonable in 32-bit mode with SSE4.1. Adding another run line to the
main vector-sext.ll test seems a better way to do that.