[Assignment Tracking][SROA] Handle createFragmentExpression failure

createFragmentExpression will fail if it determines that the expression cannot
be split over fragments. Handle this case in SROA. Similarly to D147312 this
should be a rare occurrence as the `dbg.assign` will usually reference the
`Value` being stored without modifying it with a `DIExpression`.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D147431