Improve coalescing performance by using a FxHashMap (#340)

* Use FxHashMap instead of HashMap for better performance

* Replace the binary search in the coalescing pass with a FxHashMap

This speeds up coalescing by up to 16% and overall compilation by 9%

lib/codegen/src/lib.rs

@@ -95,6 +95,7 @@ mod constant_hash;
 mod context;
 mod dce;
 mod divconst_magic_numbers;
+mod fx;
 mod iterators;
 mod legalizer;
 mod licm;