[llvm] [GlobalIsel] combine extract vector element (PR #91922)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

We should add a Combiner.rst:
* Each combine is a precise pattern in Combine.td, for compile-time and targets can pick their choice of combines
* There are no visit functions
* Document the differences of the Gisel Combiner and InstCombine, i.e., the kind of combines and aggressiveness
* The Gisel combiner is a pure legalization artefact combiner?

The DAGCombiner seems to do more than combining legalization artefacts.

https://github.com/llvm/llvm-project/pull/91922