Separate immediate and value operands in the instruction format.

Instruction formats are now identified by a signature that doesn't
include the ordering of value operands relative to immediate operands.

This means that the BinaryRev instruction format becomes redundant, so
delete it. The isub_imm instruction was the only one using that format.
Rename it to irsub_imm to make it clear what it does now that it is
printed as 'irsub_imm v2, 45'.

lib/cretonne/src/ir/instructions.rs

@@ -145,13 +145,6 @@ pub enum InstructionData {
         arg: Value,
         imm: Imm64,
     },
-    // Same as `BinaryImm`, but the immediate is the left-hand-side operand.
-    BinaryImmRev {
-        opcode: Opcode,
-        ty: Type,
-        arg: Value,
-        imm: Imm64,
-    },
     BinaryOverflow {
         opcode: Opcode,
         ty: Type,