Fix ARM issues with Containment

Fix ARM issues with Containment

These are changes that should have been part of PR dotnet/coreclr#13198:
- Correctly contain struct arguments
- Correctly get number of source registers
- Clear register assignments on `GT_FIELD_LIST` nodes (thanks @hseok-oh).
- Remove now-redundant `ContainCheck` methods for armarch targets.

Commit migrated from https://github.com/dotnet/coreclr/commit/5495b89a1368ecb279f2d1b39e69fdab96b9faa4