[X86] Don't form masked vpcmp/vcmp/vptestm operations if the setcc node has more than one use.

We're better of emitting a single compare + kand rather than a compare for the
other use and a masked compare.

I'm looking into using custom instruction selection for VPTESTM to reduce the
ridiculous number of permutations of patterns in the isel table. Putting a one
use check on all masked compare folding makes load fold matching in the custom
code easier.

llvm-svn: 358358