[Scalarizer][Bug] Only scatter/gather in extract value for trivially vectorizable intrinsics

[farzonl]

Issue

Today we scatter/gather all extract values of struct return types. Thats out of lock step with how we split the call. The fix is to check if the call was trivially vectorizable before. handling the extract element case.

Background

This is rather assert while we are doing Scatterer::operator[] and we are creating CreateExtractElement but Type is not a Vector but a Structure. Adding uadd.with.overflow to isTriviallyScalarizable is 'fixing' this.

Thats a bug. Adding to isTriviallyScalarizable is not the right fix. Whats going on is because uadd.with.overflow isn't in isTriviallyScalarizable we aren't splitting the calls so when we call Scatterer Op0 it isn't operating on the right type. A correct fix wouldn't replace extract element unless we scalarized the call.

Test case:

define <3 x i32> @test_(<3 x i32> %a, <3 x i32> %b) {
  %r = call { <3 x i32>, <3 x i1> } @llvm.uadd.with.overflow.v3i32(<3 x i32> %b, <3 x i32> %b)
  %el = extractvalue { <3 x i32>, <3 x i1> } %r, 0
  ret <3 x i32> %el
}

./build/bin/opt -passes='function(scalarizer)' ./build/test_fail.ll -S
opt: ./llvm-project/llvm/include/llvm/Support/Casting.h:578: decltype(auto) llvm::cast(From *) [To = llvm::VectorType, From = llvm::Type]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.

Originally posted by @farzonl in #111569 (comment)