[llvm] [GISel] LegalizationArtifactCombiner: Elide redundant G_SEXT_INREG (PR #93687)

[Amara Emerson via llvm-commits]

aemerson wrote:

In the original review for https://github.com/llvm/llvm-project/commit/373c343a77a7afaa07179db1754a52b620dfaf2e you'll see that there were multiple justifications stated for adding the combine to the artifact combiner. One of them was that it was a net compile time benefit and that the call to KB wouldn't be useless work most of the time.

In order to add this to the artifact combiner we'll need similar justification because strictly speaking it doesn't belong here.

https://github.com/llvm/llvm-project/pull/93687