[MSSAUpdater] Handle simplified accesses when updating phis (#78272)

This is a followup to #76819. After those changes, we can still run into
an assertion failure for a slight variation of the test case: When
fixing up MemoryPhis, we map the incoming access to the access of the
cloned instruction -- which may now no longer exist.

Fix this by reusing the getNewDefiningAccessForClone() helper, which
will look upwards for a new defining access in that case.