Add an icmp_imm instruction.

Compare a scalar integer to an immediate constant. Both Intel and RISC-V
ISAs have this operation.

This requires the addition of a new IntCompareImm instruction format.

filetests/parser/tiny.cton

@@ -54,14 +54,14 @@ function icmp(i32, i32) {
 ebb0(vx0: i32, vx1: i32):
     v0 = icmp eq, vx0, vx1
     v1 = icmp ult, vx0, vx1
-    v2 = icmp sge, vx0, vx1
+    v2 = icmp_imm sge, vx0, -12
     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:     v2 = icmp_imm sge, vx0, -12
 ; nextln:     v3 = irsub_imm vx1, 45
 ; nextln: }