[AMDGPU] Cope with SelectionDAG::UpdateNodeOperands returning a different SDNode (#65765)

SITargetLowering::adjustWritemask calls SelectionDAG::UpdateNodeOperands
to update an EXTRACT_SUBREG node in-place to refer to a new IMAGE_LOAD
instruction, before we delete the old IMAGE_LOAD instruction. But in
UpdateNodeOperands can do CSE on the fly and return a different
EXTRACT_SUBREG node, so the original EXTRACT_SUBREG node would still
exist and would refer to the old deleted IMAGE_LOAD instruction. This
caused errors like:

t31: v3i32,ch = <<Deleted Node!>> # D:1
This target-independent node should have been selected!
UNREACHABLE executed at lib/CodeGen/SelectionDAG/InstrEmitter.cpp:1209!

Fix it by detecting the CSE case and replacing all uses of the original
EXTRACT_SUBREG node with the CSE'd one.

Recommit with a fix for a use-after-free bug in the first version of
this patch (#65340) which was caught by asan.