Implement first D-specific optimization pass: -simplify-drtcalls. It uses the machinery of the standard -simplify-libcalls pass, but optimizes calls to the D runtime instead of calls to C libraries. At the moment, these optimizations are implemented by this pass: - Avoid the runtime call for `arr.length = newlen` if it can determine that the new length isn't longer than the old one. - Ditto for `cast(T[]) arr` if it will clearly always succeed. (e.g. if the length of the original array is zero, or if the old element size is a multiple of the new element size)

static foocalled = false;
static barcalled = false;
void foo() { foocalled = true; }
void bar() { barcalled = true; }

void f(bool b)
{
  return b ? foo() : bar();
}

void main()
{
  f(true);
  assert(foocalled && !barcalled);
  f(false);
  assert(foocalled && barcalled);
}