[GISel] Handle G_TRUNC in `matchExtractVecEltBuildVec`

[Pierre-vh]

Summary

Spotted some cases in D134354 where this was an issue.

[arsenm]

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

3930–3931

I don't see why this legality check (or the one below for build_vector_trunc) are here. The equivalent DAG combine does not check for the legality, and you're not creating a new one in the apply

3937

getVRegDef should be adequate here

[Pierre-vh]

getDefIgnoringCopies -> getVRegDef

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

3930–3931

@aemerson I see you implemented this one, is it okay to remove the legalizer checks?

[aemerson]

llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp

3930–3931

Sure, I don’t remember why it’s there either.

[Pierre-vh]

Remove legalizer checks, simplify code