[InstCombine] Shift amount reassociation in bittest: relax one-use check when shiftin...

[InstCombine] Shift amount reassociation in bittest: relax one-use check when shifting constant

If one of the values being shifted is a constant, since the new shift
amount is known-constant, the new shift will end up being constant-folded
so, we don't need that one-use restriction then.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368519 91177308-0d34-0410-b5e6-96231b3b80d8