Add a coalescing pass to the register allocator.

Coalescing means creating virtual registers and transforming the code
into conventional SSA form. This means that every value used as a branch
argument will belong to the same virtual register as the corresponding
EBB argument value.

Conventional SSA form makes it easy to avoid memory-memory copies when
spilling values, and the virtual registers can be used as hints when
picking registers too. This reduces the number of register moves needed
for EBB arguments.

lib/cretonne/src/regalloc/mod.rs

@@ -9,6 +9,7 @@ pub mod live_value_tracker;
 pub mod coloring;
 
 mod affinity;
+mod coalescing;
 mod context;
 mod diversion;
 mod pressure;