Enable object stack allocation in R2R mode.

Enable object stack allocation in R2R mode.

This change modified the importer to create GenTreeAllocObj node for
box and newobj instead of a helper call in R2R mode. ObjectAllocator phase
decides whether the object can be allocated on the stack or has to be created
on the heap via a helper call.

To trigger object stack allocation COMPlus_JitObjectStackAllocation has
to be set (it's not set by default).

Commit migrated from https://github.com/dotnet/coreclr/commit/3e06d9f5911ffe30894b3e08ef955f645df3acd9