[LV][ARM] Improve reduction costmodel for mismatching extension types.

Given a MLA reduction from two different types (say i8 and i16), we were
previously failing to find the reduction pattern, often making us chose
the lower vector factor. This improves that by using the largest of the
two extension types, allowing us to use the larger VF as the type of the
reduction.

As per https://godbolt.org/z/KP549EEYM the backend handles this
valiantly, leading to better performance.

Differential Revision: https://reviews.llvm.org/D115432