[llvm] Fix incorrect codegen with respect to GEPs #85333 (PR #88440)

[Paul Kirth via llvm-commits]

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ilovepi wrote:

@nikic I filed https://github.com/llvm/llvm-project/issues/77852 a while back when we ran into this, but are there other known issues of non-determinism that you can point me to? This sounds like something worth fixing, since we've found GVNHoist/GVNSink save a decent amount of size for embedded targets, and it would be ideal if we could stabilize it enough to enable in the `-Oz` pipeline by default.

https://github.com/llvm/llvm-project/pull/88440