Fix for bug 20499.

Fix for bug 20499.

When the jit is copying a struct-typed return to the return local in a synchronous method on arm64,
it ends up invoking an importer utility outside the importer, where impTreeLast is not set.

The call sequence is
fgMorphBlocks --> gtNewTempAssign --> impAssignStruct --> impAssignStructPtr --> impAppendTree

When impAssignStructPtr sees GT_COMMA src nodes, it unwraps them and inserts additional statements.
The fix is to pass an insertion point statement through this call chain to prevent impAssignStruct
and impAssignStructPtr from calling impAppendTree outside of the importer.

Commit migrated from https://github.com/dotnet/coreclr/commit/73f84776b9db118f855672724388cb1def3f2906