[x86] avoid unnecessary shuffle mask math in combineX86ShufflesRecursively()

This is a follow-up to https://reviews.llvm.org/D34174 / https://reviews.llvm.org/rL305398.

We mentioned replacing the multiplies with shifts, but the real win seems to be in
bypassing the extra ops in the common case when the RootRatio and OpRatio are one.

This gives us another 1-2% overall win for the test in PR32037:
https://bugs.llvm.org/show_bug.cgi?id=32037

llvm-svn: 305414