Immediately tag initialization code to prevent untagged spills.
When allocating new objects on the heap, we previously returned a CmmExpr containing the heap pointer as well as the tag expression, which would be added to the code graph upon first usage. Unfortunately, this meant that untagged heap pointers living in registers might be spilled to the stack, where they interacted poorly with garbage collection (we saw this bug specifically with the compacting garbage collector.) This fix immediately tags the register containing the heap pointer, so that unless we have extremely unfriendly spill code, the new pointer will never be spilled to the stack untagged. An alternate solution might have been to modify allocDynClosure to tag the pointer upon the initial register allocation, but not all invocations of allocDynClosure tag the resulting pointer, and threading the consequent CgIdInfo for the cases that did would have been annoying.