[InstCombine] Infer nusw + nneg -> nuw for getelementptr (#111144)

If the gep is nusw (usually via inbounds) and the offset is
non-negative, we can infer nuw.

Proof: https://alive2.llvm.org/ce/z

[InstCombine] Infer nusw + nneg -> nuw for getelementptr (#111144)

If the gep is nusw (usually via inbounds) and the offset is
non-negative, we can infer nuw.

Proof: https://alive2.llvm.org/ce/z/ihztLy

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[InstCombine] Preserve inbounds when canonicalizing gep+add (#90160)

When canonicalizing gep+add into gep+gep we can preserve inbounds if the
add is also nsw and both add operands are non-negative

[InstCombine] Preserve inbounds when canonicalizing gep+add (#90160)

When canonicalizing gep+add into gep+gep we can preserve inbounds if the
add is also nsw and both add operands are non-negative (or both
negative, but I don't think that's practically relevant).

Proof: https://alive2.llvm.org/ce/z/tJLBta

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[InstCombine] Add tests for inbounds preservation in gep+add fold (NFC)

[InstCombine] Canonicalize constant GEPs to i8 source element type (#68882)

This patch canonicalizes getelementptr instructions with constant
indices to use the `i8` source element type. This makes

[InstCombine] Canonicalize constant GEPs to i8 source element type (#68882)

This patch canonicalizes getelementptr instructions with constant
indices to use the `i8` source element type. This makes it easier for
optimizations to recognize that two GEPs are identical, because they
don't need to see past many different ways to express the same offset.

This is a first step towards
https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699.
This is limited to constant GEPs only for now, as they have a clear
canonical form, while we're not yet sure how exactly to deal with
variable indices.

The test llvm/test/Transforms/PhaseOrdering/switch_with_geps.ll gives
two representative examples of the kind of optimization improvement we
expect from this change. In the first test SimplifyCFG can now realize
that all switch branches are actually the same. In the second test it
can convert it into simple arithmetic. These are representative of
common optimization failures we see in Rust.

Fixes https://github.com/llvm/llvm-project/issues/69841.

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[InstCombine] Support zext nneg in gep of sext add fold

Add m_NNegZext() and m_SExtLike() matchers to make doing these kinds
of changes simpler in the future.

[InstCombine] Add zext nneg test variant for gep of sext add fold (NFC)

[InstCombine] Canonicalise SextADD + GEP (#69581)