[x86] replace div/rem with shift/mask for better shuffle combining perf

We know that shuffle masks are power-of-2 sizes, but there's no way (?) for LLVM to know that, 
so hack combineX86ShufflesRecursively() to be much faster by replacing div/rem with shift/mask.

This makes the motivating compile-time bug in PR32037 ( https://bugs.llvm.org/show_bug.cgi?id=32037 ) 
about 9% faster overall.

Differential Revision: https://reviews.llvm.org/D34174

llvm-svn: 305398