[LV] Teach LoopVectorizationLegality about struct vector calls (#119221)

This is a split-off from #109833 and only adds code relating to checking
if a struct-returning call can be vectorized.

Th

[LV] Teach LoopVectorizationLegality about struct vector calls (#119221)

This is a split-off from #109833 and only adds code relating to checking
if a struct-returning call can be vectorized.

This initial patch only allows the case where all users of the struct
return are `extractvalue` operations that can be widened.

```
%call = tail call { float, float } @foo(float %in_val)
%extract_a = extractvalue { float, float } %call, 0
%extract_b = extractvalue { float, float } %call, 1
```

Note: The tests require the VFABI changes from #119000 to pass.

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[VFABI] Add support for vector functions that return struct types (#119000)

This patch updates the `VFABIDemangler` to support vector functions that
return struct types. For example, a vector varia

[VFABI] Add support for vector functions that return struct types (#119000)

This patch updates the `VFABIDemangler` to support vector functions that
return struct types. For example, a vector variant of `sincos` that
returns a vector of sine values and a vector of cosine values within a
struct.

This patch also adds some helpers for vectorizing types (including
struct types). Some of these are used in the `VFABIDemangler`, and
others will be used in subsequent patches, so this patch simply adds
tests for them.

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