Propagate constants more

[JulianKemmerer]

Ex.

point.x = 1;
point.y = 2;
other_point = point;
//return point.x  + point.y; // Does correctly resolve to constant
// Does not resolve constant through 'other_point' struct compound reference
return other_point.x + other_point.y;

// Pseudo code
uint foo(x,y)
{
  return x + y;
}
a = foo(1,2); // Does not resolve to constant

Same for bit manipulation and a bunch of math functions. And finally 'null' all-zeros values too.

[JulianKemmerer]

Very much related to #65

[JulianKemmerer]

As of closing #65

The constant value of the expression is not resolved. But it is known that things downstream/derived from it are constant too (not part of pipeline registers,etc)

point.x = 1;
point.y = 2;
other_point = point;
//return point.x  + point.y; // Does correctly resolve to constant
// Does not resolve constant through 'other_point' struct compound reference
return other_point.x + other_point.y;
^ As of Issue #65 close 'other_point.x + other_point.y' is known to be constant (value not known though)

[JulianKemmerer]

Note to check for use of global vars when checking the constant-ness of funcs