[llvm] [SeparateConstOffsetFromGEP] Extend handling of ExtractBase (PR #163241)

[Jeffrey Byrnes via llvm-commits]

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jrbyrnes wrote:

Somewhat -- the main usecase for this that I'm aware of is simplifying `GEPOperator`s indexing structs.

`accumulateByteOffset` does not accept `GEPOperator`s (only `GEPInstruction`s), nor does it handle struct types by default.

It seemed cleaner to add new handling rather than special casing the existing handling.

https://github.com/llvm/llvm-project/pull/163241