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/write.rs

@@ -237,7 +237,6 @@ fn write_instruction(w: &mut Write,
         UnarySplit { arg, .. } => writeln!(w, " {}", arg),
         Binary { args, .. } => writeln!(w, " {}, {}", args[0], args[1]),
         BinaryImm { arg, imm, .. } => writeln!(w, " {}, {}", arg, imm),
-        BinaryImmRev { imm, arg, .. } => writeln!(w, " {}, {}", imm, arg),
         BinaryOverflow { args, .. } => writeln!(w, " {}, {}", args[0], args[1]),
         Ternary { args, .. } => writeln!(w, " {}, {}, {}", args[0], args[1], args[2]),
         TernaryOverflow { ref data, .. } => writeln!(w, " {}", data),