Fix constant folding for simple binary expression

Fix constant folding for simple binary expression

This change improves constant folding of numeric and
string literals.

Also, we go back generating quadruples instead of
IR-tree-expressions. This is kind of needed to reduce register
pressure in the V4 VM. That is, V4 has typed registers so before
reusing a register we need to look at the type and eventually
dispose its contents (e.g. when a QString or a QUrl is stored
in the register). Unfortunately, we can't effort to have all
these checks in the V4 instructions. So we change `binop'
to generate literals (e.g. CONST or STRING) or a TEMP (aka
a preassigned register that cannot be reused).

For exmaple, the IR code generated for

  Rectangle {
    color: "b" + "l" + ("u" + "e")
    width: 10 + 20 + 30
  }

is

====================
	line: 3	column: 10
L0x811ca10:
	t0 = string_to_color("blue");
	return t0;

====================
	line: 4	column: 10
L0x811ca50:
	return 60;

Change-Id: I4d8482ddab9193d8469bda6461bfb2e5a3eeb197
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>