[LLVM][IR] Support target extension types in vectors (#140630)

This change is to support target extension types in vectors. The change
allows sized target extension types to opt-in to being a valid vector
element.
Allowing target extension types as vector elements will allow backends
to use vector operations such as `insertelement` and `extractelement` on
their target types with minimal changes.

RFC:
https://discourse.llvm.org/t/rfc-supporting-sized-target-extension-types-in-vector/86431

llvm/docs/LangRef.rst

@@ -4438,7 +4438,8 @@ the type size is smaller than the type's store size.
       < vscale x <# elements> x <elementtype> > ; Scalable vector
 
 The number of elements is a constant integer value larger than 0;
-elementtype may be any integer, floating-point or pointer type. Vectors
+elementtype may be any integer, floating-point, pointer type, or a sized  
+target extension type that has the ``CanBeVectorElement`` property. Vectors
 of size zero are not allowed. For scalable vectors, the total number of
 elements is a constant multiple (called vscale) of the specified number
 of elements; vscale is a positive integer that is unknown at compile time