Generate an InstBuilder trait.

All of the instruction format an opcode methods are emitted as an
InstBuilder trait instead of adding them to the Bulder struct directly.
The methods only make use of the InstBuilderBase methods to create new
instructions.

This makes it possible to reuse the InstBuilder trait for different ways
of inserting instructions.

lib/cretonne/src/cfg.rs

@@ -139,7 +139,7 @@ impl ControlFlowGraph {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use ir::{Function, Builder, Cursor, VariableArgs, types};
+    use ir::{Function, Builder, InstBuilder, Cursor, VariableArgs, types};
 
     #[test]
     fn empty() {