[InstCombine] Infer nsw/nuw for trunc (#87910)

This patch adds support for inferring trunc's nsw/nuw flags.

[InstCombine] canonicalize insertelement order based on index

This puts lower insert indexes before higher. This is independent
of endian, so it requires an adjustment to a fold added with
4446f71ce

[InstCombine] canonicalize insertelement order based on index

This puts lower insert indexes before higher. This is independent
of endian, so it requires an adjustment to a fold added with
4446f71ce392, but it makes that fold more robust.
That's also where this patch was suggested - D139668.

This matches what we already do in DAGCombiner, but there is one
more constraint because there's an existing canonicalization for
insert-of-scalar-constant. I'm not sure if that is still needed,
so it may be adjusted/removed as a follow-up.

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[InstCombine] limit pair-of-insertelement folds to avoid miscompile

This transform was added with 4446f71ce392. However, as noted in
the post-commit feedback, the transform is not safe with an
arbit

[InstCombine] limit pair-of-insertelement folds to avoid miscompile

This transform was added with 4446f71ce392. However, as noted in
the post-commit feedback, the transform is not safe with an
arbitrary base vector because we may leak poison from a narrow
element into an adjacent element when bitcasting.

I made the least invasive code change in case we do figure out
a way to make this safe.

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[InstCombine] try to fold a pair of insertelements into one insertelement

This replaces patches that tried to convert related patterns to shuffles
(D138872, D138873, D138874 - reverted/abandoned) bu

[InstCombine] try to fold a pair of insertelements into one insertelement

This replaces patches that tried to convert related patterns to shuffles
(D138872, D138873, D138874 - reverted/abandoned) but caused codegen
problems and were questionable as a canonicalization because an
insertelement is a simpler op than a shuffle.

This detects a larger pattern -- insert-of-insert -- and replaces with
another insert, so this hopefully does not cause any problems.

As noted by TODO items in the code and tests, this could go a lot further.
But this is enough to reduce the motivating test from issue #17113.

Example proofs:
https://alive2.llvm.org/ce/z/NnUv3a

I drafted a version of this for AggressiveInstCombine, but it seems that
would uncover yet another phase ordering gap. If we do generalize this to
handle the full range of potential patterns, that may be worth looking at
again.

Differential Revision: https://reviews.llvm.org/D139668

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[InstCombine] add tests for insertelts of truncs; NFC