Followup work to bug 979594. We can generate better code / lower register pressure by embedding small immediates in the instructions rather than forcing them into register, as we do now. Known cases: - When the value operand to the atomic binary operations is a small immediate - When the oldval operand to the atomic compareExchange operation is a small immediate.

Memo to self: I have incomplete patches for this but it's a fair amount of churn up the entire stack for little gain (and the register pressure argument is dubious because a new temp is needed elsewhere); deprioritize for now.