[InstCombine] Shift amount reassociation in bittest: drop pointless one-use restriction

[InstCombine] Shift amount reassociation in bittest: drop pointless one-use restriction

That one-use restriction is not needed for correctness - we have already
ensured that one of the shifts will go away, so we know we won't increase
the instruction count. So there is no need for that restriction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368518 91177308-0d34-0410-b5e6-96231b3b80d8