[AMDGPU] Prevent re-visits in LowerBufferFatPointers (#159168)
Fixes https://github.com/iree-org/iree/issues/22001
The visitor in SplitPtrStructs would re-visit instructions if an
instruction earlier in program order caused a recursive visit() call via
getPtrParts(). This would cause instructions to be processed multiple
times.
As a consequence of this, PHI nodes could be added to the Conditionals
array multiple times, which would to a conditinoal that was already
simplified being processed multiple times. After the code moved to
InstSimplifyFolder, this re-processing, combined with more agressive
simplifications, would lead to an attempt to replace an instruction with
itself, causing an assertion failure and crash.
This commit resolves the issue and adds the reduced form of the crashing
input as a test.