Don't assume that a left-shift of a value with one bit set will have

Don't assume that a left-shift of a value with one bit set will have
one bit set, because the bit may be shifted off the end. Instead,
just check for a constant 1 being shifted. This is still sufficient
to handle all the cases in test/CodeGen/X86/bt.ll. This fixes PR3583.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64622 91177308-0d34-0410-b5e6-96231b3b80d8