[GVN] Fix MemorySSA update when replacing assume(false) with stores.

When replacing an assume(false) with a store, we have to be more careful
with the order we insert the new access. This patch updates the code to
look at the accesses in the block to find a suitable insertion point.

Alterantively we could check the defining access of the assume, but IIRC
there has been some discussion about making assume() readnone, so
looking at the access list might be more future proof.

Fixes PR48072.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D90784