[Local] Use nusw and nuw flags in emitGEPOffset()

[IR] Fix GEP offset computations for vector GEPs (#75448)

Vectors are always bit-packed and don't respect the elements' alignment
requirements. This is different from arrays. This means offsets of

[IR] Fix GEP offset computations for vector GEPs (#75448)

Vectors are always bit-packed and don't respect the elements' alignment
requirements. This is different from arrays. This means offsets of
vector GEPs need to be computed differently than offsets of array GEPs.

This PR fixes many places that rely on an incorrect pattern
that always relies on `DL.getTypeAllocSize(GTI.getIndexedType())`.
We replace these by usages of `GTI.getSequentialElementStride(DL)`,
which is a new helper function added in this PR.

This changes behavior for GEPs into vectors with element types for which
the (bit) size and alloc size is different. This includes two cases:

* Types with a bit size that is not a multiple of a byte, e.g. i1.
GEPs into such vectors are questionable to begin with, as some elements
are not even addressable.
* Overaligned types, e.g. i16 with 32-bit alignment.

Existing tests are unaffected, but a miscompilation of a new test is fixed.

---------

Co-authored-by: Nikita Popov <github@npopov.com>

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[llvm][TypeSize] Fix addition/subtraction in TypeSize. (#72979)

It seems TypeSize is currently broken in the sense that:

TypeSize::Fixed(4) + TypeSize::Scalable(4) => TypeSize::Fixed(8)

with

[llvm][TypeSize] Fix addition/subtraction in TypeSize. (#72979)

It seems TypeSize is currently broken in the sense that:

TypeSize::Fixed(4) + TypeSize::Scalable(4) => TypeSize::Fixed(8)

without failing its assert that explicitly tests for this case:

assert(LHS.Scalable == RHS.Scalable && ...);

The reason this fails is that `Scalable` is a static method of class
TypeSize,
and LHS and RHS are both objects of class TypeSize. So this is
evaluating
if the pointer to the function Scalable == the pointer to the function
Scalable,
which is always true because LHS and RHS have the same class.

This patch fixes the issue by renaming `TypeSize::Scalable` ->
`TypeSize::getScalable`, as well as `TypeSize::Fixed` to
`TypeSize::getFixed`,
so that it no longer clashes with the variable in
FixedOrScalableQuantity.

The new methods now also better match the coding standard, which
specifies that:
* Variable names should be nouns (as they represent state)
* Function names should be verb phrases (as they represent actions)

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[InstCombine] Handle more scalable geps in EmitGEPOffset (#71699)

Following up on #71565, this makes scalable splats in EmitGEPOffset use
the ElementCount as opposed to assuming it is fixed width,

[InstCombine] Handle more scalable geps in EmitGEPOffset (#71699)

Following up on #71565, this makes scalable splats in EmitGEPOffset use
the ElementCount as opposed to assuming it is fixed width, and attempts
to handle scalable offsets with vector geps by splatting the vscale to
each vector lane.

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[InstCombine] Handle scalable geps in EmitGEPOffset (#71565)

This adds scalable handling for scalable vectors in emitGEPOffset. This
was noticed in some tests that Biplob was creating, so might be

[InstCombine] Handle scalable geps in EmitGEPOffset (#71565)

This adds scalable handling for scalable vectors in emitGEPOffset. This
was noticed in some tests that Biplob was creating, so might be unlikely
to come up much in practice. I've attempted to add test coverage for
various places EmitGEPOffset is called. The vscale intrinsics will
currently emit multiple copies, relying on later CSE to combine them.

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[Local] Merge constant / non-constant code paths (NFC)

Let IRBuilder perform the constant folding. Avoids some uses of
ConstantExpr.

Detemplate llvm::EmitGEPOffset and move it into a cpp file. NFC.