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.
When the CRETONNE_DBG environment variable is set, send debug messages
to a file named cretonne.dbg.*.
The trace facility is only enabled when debug assertions are on.
