[mlir][memref] Fix segfault in SROA (#71063)

Fixes #70902.

The out of bounds check in the SROA implementation for MemRef was not
actually testing anything because it only operated on a store op which
does not trigger the logic by itself. It is now checked for real and the
underlying bug is fixed.

I checked the LLVM implementation just in case but this should not
happen as out-of-bound checks happen in GEP's verifier there.