It's not that easy. If we're only checking -fsanitize=shift-base we
still need to verify that exponent has sane value, otherwise
UBSan-inserted checks for base will contain undefined behavior
themselves.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@231409 91177308-0d34-0410-b5e6-96231b3b80d8