Re-run of loop pass

[Graham]

Micheal

There is a small problem with your patch it doesn't
handle approx_final_value() returning 0 which happens
for an EQ.

The fix is to change 

	if (REG_P (initial_value))

into

	if (final_value != NULL_RTX && REG_P (initial_value))

The problem only triggered when I compiled gnu_regex.c from gdb-4.17
on ix86-linux.

Graham