Less Conservative GtObj

Less Conservative GtObj

Don't mark GT_OBJ with GTF_EXCEPT or GTF_GLOB_REF if it is a local struct.
Also, fix changes needed in arm register allocation and code generation to handle the case where a struct argument is promoted, which appears to be code that was never exercised.
These changes are in preparation for further changes for First Class Structs. I am working to replace block ops with assignments, and was attempting to get to zero diffs, but it is quite difficult to do so given some inconsistencies in the treatment of GT_OBJs of locals.

Commit migrated from https://github.com/dotnet/coreclr/commit/74b27523b5a150e4cac25d43b37efbbcad008020