Coalesce some formats into MultiAry.

Allow some flexibility in the signature matching for instruction
formats. In particular, look for a value list format as a second chance
option.

The Return, ReturnReg, and TernaryOverflow formats all fit the single
MultiAry catch-all format for instructions without immediate operands.

lib/cretonne/src/ir/builder.rs

@@ -3,7 +3,7 @@
 //! A `Builder` provides a convenient interface for inserting instructions into a Cretonne
 //! function. Many of its methods are generated from the meta language instruction definitions.
 
-use ir::{types, instructions};
+use ir::types;
 use ir::{InstructionData, DataFlowGraph, Cursor};
 use ir::{Opcode, Type, Inst, Value, Ebb, JumpTable, SigRef, FuncRef, ValueList};
 use ir::immediates::{Imm64, Uimm8, Ieee32, Ieee64};