DynASM/x64: Add VREG support to mov64.

Thanks to Peter Cawley.

dynasm/dasm_x86.lua

@@ -1678,7 +1678,7 @@ if x64 then
   function map_op.mov64_2(params)
     if not params then return { "reg, imm", "reg, [disp]", "[disp], reg" } end
     if secpos+2 > maxsecpos then wflush() end
-    local opcode, op64, sz, rex
+    local opcode, op64, sz, rex, vreg
     local op64 = match(params[1], "^%[%s*(.-)%s*%]$")
     if op64 then
       local a = parseoperand(params[2])
@@ -1699,11 +1699,17 @@ if x64 then
 	  werror("bad operand mode")
 	end
 	op64 = params[2]
-	opcode = 0xb8 + band(a.reg, 7) -- !x64: no VREG support.
+	if a.reg == -1 then
+	  vreg = a.vreg
+	  opcode = 0xb8
+	else
+	  opcode = 0xb8 + band(a.reg, 7)
+	end
 	rex = a.reg > 7 and 9 or 8
       end
     end
     wputop(sz, opcode, rex)
+    if vreg then waction("VREG", vreg); wputxb(0) end
     waction("IMM_D", format("(unsigned int)(%s)", op64))
     waction("IMM_D", format("(unsigned int)((%s)>>32)", op64))
   end