[Scalarizer] InsertElement handling w/ variable insert index (PR46524)

Summary:
I'm interested in taking the original C++ input,
for which we currently are stuck with an alloca
and producing roughly the lower IR,
with neither an alloca nor a vector ops:
https://godbolt.org/z/cRRWaJ

For that, as intermediate step, i'd to somehow perform scalarization.
As per @arsenmn suggestion, i'm trying to see if scalarizer can help me
avoid writing a bicycle.

I'm not sure if it's really intentional that variable insert is not handled currently.
If it really is, and is supposed to stay that way (?), i guess i could guard it..

See [[ https://bugs.llvm.org/show_bug.cgi?id=46524 | PR46524 ]].

Reviewers: bjope, cameron.mcinally, arsenm, jdoerfert

Reviewed By: jdoerfert

Subscribers: arphaman, uabelho, wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82961