Added support for Immediate mode 16 bit

Currently only afi instruction is encoded, will be adding other instructions too. For encoding we are running out of characters so was planning to append the complete modes (RXa or rxa) which one do you think is better, and also thinking of just adding remaining instruction modes as well, which we don't support as of now. Let me know if you want me to add those, or we will wait for sometime before we add those.

dynasm/dasm_s390x.lua

@@ -291,6 +291,9 @@ local function is_int32(num)
   return -2147483648 <= num and num < 2147483648
 end
 
+local function_is_int16(num)
+  return -32768 <= num and num < 32768
+
 -- Split a memory operand of the form d(b) or d(x,b) into d, x and b.
 -- If x is not specified then it is 0.
 local function split_memop(arg)
@@ -381,6 +384,18 @@ local function parse_imm(arg)
   end
 end
 
+local function parse_imm16(arg)
+  local imm_val = tonumber(arg,16)
+  if imm_val then
+    if not is_int16(imm_val) then
+      werror("Immediate value out of range: ", imm_val)
+    end
+    wputhw(imm_val)
+  else
+    waction("IMM16", nil, arg)
+  end
+end
+
 local function parse_label(label, def)
   local prefix = sub(label, 1, 2)
   -- =>label (pc label reference)
@@ -449,6 +464,7 @@ map_op = {
   adbr_2 =	"0000b31a0000h",
   aebr_2 =	"0000b30a0000h",
   ah_2 =	"00004a000000j",
+  ahi_2 =	"0000a70a0000i",
   ahy_2 =	"e3000000007al",
   afi_2 =	"c20900000000n",
   agfi_2 =	"c20800000000n",
@@ -1028,6 +1044,10 @@ local function parse_template(params, template, nparams, pos)
     elseif p == "h" then
       op2 = op2 + shl(parse_gpr(params[1]),4) + parse_gpr(params[2])
       wputhw(op1); wputhw(op2)
+    else if p == "i" then
+      op1 = op1 + shl(parse_gpr(params[1]),4)
+      wputhw(op1);
+      parse_imm16(params[2])
     elseif p == "j" then
       local d, x, b, a = parse_mem_bx(params[2])
       op1 = op1 + shl(parse_gpr(params[1]), 4) + x