[NFC][InstCombine] Better commutative tests for "shift amount reassociation in bittest" pattern.

As discussed in https://reviews.llvm.org/D63829
*if* *both* shifts are one-use, we'd most likely want to produce `lshr`,
and not rely on ordering.

Also, there should likely be a *separate* fold to do this reordering.

llvm-svn: 364772