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.

cranelift/docs/langref.rst

@@ -678,6 +678,7 @@ Integer operations
 ------------------
 
 .. autoinst:: icmp
+.. autoinst:: icmp_imm
 .. autoinst:: iadd
 .. autoinst:: iadd_imm
 .. autoinst:: iadd_cin