While doing the memory SSA work, I started wondering about V_MUST_DEFs. I think we don't need them anymore.
This may be a bit hasty on my part, I have not really thought it through. But I cannot think of a single transformation that absolutely *requires* V_MUST_DEFs to work. When they were introduced, they would help propagation and DCE in cases like: # a_5 = V_MUST_DEF <a_4>; a = 4; # a_6 = V_MUST_DEF <a_5>; a = 1; where 'a' is a non-escaping addressable local. However, DSE will/should also detect this chain, even if we used preserving defs (V_MAY_DEF). This could also be handled with memory SSA by not chaining these two stores. The propagators will happily propagate stores into V_MAY_DEFs, they do not require V_MUST_DEFs to work. Thoughts?
