[X86] Improve some shuffle decoding code to remove a conditional from a loop...

[X86] Improve some shuffle decoding code to remove a conditional from a loop and reduce the number of temporary variables. NFCI

The NumControlBits variable was definitely sketchy. I think that only worked because the expected value was 1 or 2 and the number of lanes was 2 or 4. Had their been 8 lanes the number of bits should have been 3 not 4 as the previous code would have given.

llvm-svn: 334258