Local value numbering for MIR

[pcwalton]

Local value numbering, local to a basic block, doesn't require SSA form, unlike global value numbering. We could straightforwardly run this optimization on MIR. In many cases, this would subsume constant and copy propagation, and it would also let us do nice things like detect commutativity and fold a + b and b + a together. It may be possible to do things that LLVM won't, due to it being much easier to prove & and &muts disjoint when done on MIR.

[pcwalton]

Oh, by the way, one nice thing about this is that it could be upgraded to global value numbering later if MIR grows an SSA form.

[pcwalton]

WIP branch is at https://github.com/pcwalton/rust/tree/value-numbering — it can CSE simple test cases.