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'.

filetests/parser/tiny.cton

@@ -55,12 +55,14 @@ ebb0(vx0: i32, vx1: i32):
     v0 = icmp eq, vx0, vx1
     v1 = icmp ult, vx0, vx1
     v2 = icmp sge, vx0, vx1
+    v3 = irsub_imm vx1, 45
 }
 ; sameln: function icmp(i32, i32) {
 ; nextln: ebb0(vx0: i32, vx1: i32):
 ; nextln:     v0 = icmp eq, vx0, vx1
 ; nextln:     v1 = icmp ult, vx0, vx1
 ; nextln:     v2 = icmp sge, vx0, vx1
+; nextln:     v3 = irsub_imm vx1, 45
 ; nextln: }
 
 ; Floating condition codes.