[LSRA][RyuJIT] Consider merging allocating free & busy regs #9399
Description
Currently, LSRA iterates over all the free registers, and then iterates again if it has to find a register to spill. It would probably be more efficient, as well as more effective, to combine these passes. In particular, it would avoid the situation where we make a bad register choice for a critical Interval simply because it is the only free register, when it may be better to spill a less important Interval.
In particular, it seems like this may be the best approach to address #8846.
category:cq
theme:register-allocator
skill-level:expert
cost:medium