Reorganize the instruction builder traits.

Leave the primary InstBuilderBase trait alone, but add an alternative
InstInserterBase trait that can be implemented instead by builders that
always allocate new instructions with dfg.make_inst().

Any implementation of InstInserterBase can be used as an instruction
builder by wrapping it in an InsertBuilder. The InsertBuilder type adds
additional functionality via the with_results() method which makes it
possible to override the result values on the instruction that is built.

The motivation for this shuffle is that the with_result() functionality
can now be reused by different kinds of instruction builders, as long as
they insert new instructions. So ReplaceBuilder doesn't get
with_results().

lib/cretonne/src/ir/dfg.rs

@@ -5,7 +5,7 @@ use isa::TargetIsa;
 use ir::builder::{InsertBuilder, ReplaceBuilder};
 use ir::extfunc::ExtFuncData;
 use ir::instructions::{Opcode, InstructionData, CallInfo};
-use ir::layout::Cursor;
+use ir::layout::{Cursor, LayoutCursorInserter};
 use ir::types;
 use ir::{Ebb, Inst, Value, Type, SigRef, Signature, FuncRef, ValueList, ValueListPool};
 use write::write_operands;
@@ -486,8 +486,8 @@ impl DataFlowGraph {
     /// Create an `InsertBuilder` that will insert an instruction at the cursor's current position.
     pub fn ins<'c, 'fc: 'c, 'fd>(&'fd mut self,
                                  at: &'c mut Cursor<'fc>)
-                                 -> InsertBuilder<'c, 'fc, 'fd> {
-        InsertBuilder::new(self, at)
+                                 -> InsertBuilder<'fd, LayoutCursorInserter<'c, 'fc, 'fd>> {
+        InsertBuilder::new(LayoutCursorInserter::new(at, self))
     }
 
     /// Create a `ReplaceBuilder` that will replace `inst` with a new instruction in place.