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

This is similar to 373c343a, but for targets with zero-or-negative-one
booleans.
The difference in tests is mostly due to G_SEXT_INREG being illegal for
some targets, in which case it gets expanded into G_SHL/G_ASHR pair,
which is not currently optimized by the combiner.