Add FIXME note to backward threader.

Add FIXME note to backward threader.

There's a limitation in the path discovery bits in the backward
threader that I ran into recently and I'd like to document it so we
don't lose track of it.

Basically we stop looking if we find a threadable path through a PHI,
without taking into account that there could be multiple
paths through a PHI that resolve the path.  For example:

	x_5 = PHI <10(4), 20(5), ...>
	if (x_5 > 5)

I don't remember how we ended up skipping this, but it could existing
behavior from the old bits.  It probably skipped multiple threads
through a PHI since the generic copier couldn't re-using existing
threading paths anyhow.

Documenting for later fixing.

gcc/ChangeLog:

	* tree-ssa-threadbackward.c (back_threader::resolve_phi): Add
	FIXME note.