[release/6.0] Fix incorrect SIMD temp allocation for Vector256 with AVX2 disabled...

[release/6.0] Fix incorrect SIMD temp allocation for Vector256 with AVX2 disabled (#58850)

* Fix incorrect SIMD temp allocation for Vector256 with AVX2 disabled

The NI_Vector256_GetElement intrinsic, in some situations, requires
a stack temporary. With AVX2 disabled, this temporary was getting
allocated as a TYP_SIMD16 instead of a TYP_SIMD32, leading to overwriting
the local variable.

Add a type argument to the temp variable allocation, and allocate the
temp as the largest sized type required by any use.

Fixes #58295

* Code review change: improve arm64 SIMD temp creation type

Co-authored-by: Bruce Forstall <brucefo@microsoft.com>