Revisit the handling of SIMD types during crossgen

[CarolEidt]

See the comments on PR dotnet/coreclr#17543.

During crossgen, the JIT sets featureSIMD to false but continues to generate code for calls to fixed-size SIMD types. While this is correct, it means that any use of these types in crossgen'd code is quite inefficient.

On x86/x64 this may have some actual benefit due to the fact that we don't know whether the JIT'd code that is generated at runtime will use AVX or SSE encodings. However, this is somewhat inconsistent, as we generate SSE encodings for other operations.

On Arm64, a change was made (#14633) to enable SIMD during crossgen. However, the importing of static field helpers is not calling impNormStructType() which led to #9799. Adding a call to impNormStructType() here: https://github.com/dotnet/coreclr/blob/master/src/jit/importer.cpp#L13570 when lclType == TYP_STRUCT seemed to fix that issue.

We should consider whether to add support for fixed- or known- size SIMD types during crossgen, across architectures.

category:cq
theme:vector-codegen
skill-level:intermediate
cost:small
impact:medium

[CarolEidt]

As much as I'd like to see us improve this for 3.0, I think it will have to be deferred to future.