[MergeICmps] Ignore clobbering instructions before the loads

This is another followup to D106591. Even if there is an
instruction that clobbers one of the loads, this doesn't matter if
it happens before the loads. Those instructions aren't affected by
the transform at all.

The gep-references-bb.ll is modified to preserve the spirit of the
test, as the store to @g no longer impacts the transform.

Differential Revision: https://reviews.llvm.org/D108782