Revert "[UBSan] Split -fsanitize=shift into -fsanitize=shift-base and -fsanitize=shift-exponent."

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.

llvm-svn: 231409