Add a branch relaxation pass for #72.

Compute exact EBB header offsets and check that branches are in range.

Not implemented yet: Relax branches that are not in range.

Invoke the relax_branches() pass from the 'test binemit' file tests so
they can verify the proper encoding of branch instructions too.

lib/cretonne/src/binemit/mod.rs

@@ -3,6 +3,10 @@
 //! The `binemit` module contains code for translating Cretonne's intermediate representation into
 //! binary machine code.
 
+mod relaxation;
+
+pub use self::relaxation::relax_branches;
+
 use ir::{Ebb, FuncRef, JumpTable, Function, Inst};
 
 /// Offset in bytes from the beginning of the function.