[X86] Remove check that avoids creating PMULDQ with illegal types. Rely on...

[X86] Remove check that avoids creating PMULDQ with illegal types. Rely on SplitOpsAndApply to legalize it.

Create PMULDQ/PMULUDQ as long as the number of elements is a power of 2.

This seems to give some improvements in our ability to use SimplifyDemandedBits.

llvm-svn: 350084