Set the size of SIMD this pointer

Set the size of SIMD this pointer

When setting the SIMD type of a "this" pointer,
we need to capture the size of the SIMD type.
This was previously being set in a method in simd.cpp,
but when the SIMD type recognition was moved
earlier in the JIT, it was omitted.
This caused an assert in the case where a fixed-size
SIMD type is handled by `fgMorphCombineSIMDFieldAssignments()`.

Fixes bug 288222

Commit migrated from https://github.com/dotnet/coreclr/commit/13d9b9f08cebb57567838b1ddf1c4f4477961e50