[MSSA] Don't require clone creation to succeed (#76819)

Sometimes, we create a MemoryAccess for an instruction, which is later
simplified (e.g. via devirtualization) such that the new instruction has
no memory effects anymore.

If we later clone the instruction (e.g. during unswitching), then MSSA
will not create a MemoryAccess for the new instruction, triggering an
assert.

Disable the assertion (by passing CreationMustSucceed=false) and adjust
getDefiningAccessForClone() to work correctly in that case.

This PR implements the alternative suggestion by alinas from
https://github.com/llvm/llvm-project/pull/76142.