Add a transformation pass which removes phi nodes to which it can demonstrate

that only one value ever flows.  Has been observed to improve generated code
run times by up to 8%.  Compilation cost increases by about 0.6%, but up to 7%
total cost has been observed to be saved; iow it can be a significant win in
terms of compilation time, overall.

cranelift/codegen/src/lib.rs

@@ -105,6 +105,7 @@ mod postopt;
 mod predicates;
 mod redundant_reload_remover;
 mod regalloc;
+mod remove_constant_phis;
 mod result;
 mod scoped_hash_map;
 mod simple_gvn;