Add initial scalar FP operations (addss, subss, etc) to x64 backend.

Adds support for addss and subss. This is the first lowering for
sse floating point alu and some move operations. The changes here do
some renaming of data structures and adds a couple of new ones
to support sse specific operations. The work done here will likely
evolve as needed to support an efficient, inituative, and consistent
framework.

cranelift/codegen/src/lib.rs

@@ -1,5 +1,4 @@
 //! Cranelift code generation library.
-
 #![deny(missing_docs, trivial_numeric_casts, unused_extern_crates)]
 #![warn(unused_import_braces)]
 #![cfg_attr(feature = "std", deny(unstable_features))]