SelectionDAG: fix logic for promoting shift types

SelectionDAG: fix logic for promoting shift types

r238503 fixed the problem of too-small shift types by promoting them
during legalization, but the correct solution is to promote only the
operands that actually demand promotion.

This fixes a crash on an out-of-tree target caused by trying to
promote an operand that can't be promoted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238632 91177308-0d34-0410-b5e6-96231b3b80d8