[X86] Remove some check-prefixes from avx512-cvt.ll to prepare for an upcoming patch.

The update script sometimes has trouble when there are check-prefixes representing every possible combination of feature flags. I have a patch where the update script was generating something that didn't pass lit.

This patch just removes some check-prefixes and expands out some of the checks to workaround this.

llvm-svn: 324819