Fix nested struct field assignment emission

[DavidObando]

Summary

• emit expression-based value-type field receivers through managed addresses
• plan spill locals for non-addressable struct rvalue receivers
• add executing regressions for nested class-held structs and struct rvalues

Root cause

Expression-based field assignments always emitted receiver values before stfld. Nested struct receivers therefore used ldfld, pushing a copy where CLR requires a managed pointer. Routing through existing instance-receiver addressability logic emits ldflda for addressable field chains and ldloca for spilled rvalues.

Validation

• dotnet test test/Compiler.Tests/Compiler.Tests.csproj --no-restore --filter FullyQualifiedName~FieldAssignmentEmitTests (10 passed)
• related emitter regressions (47 passed)
• dotnet test test/Compiler.Tests/Compiler.Tests.csproj --no-restore (3551 passed)

Closes #2818