combine: Don't call extract_left_shift with count < 0 (PR67483)

combine: Don't call extract_left_shift with count < 0 (PR67483)

If the compiled program does a shift by a negative amount, combine will
happily work with that, but it shouldn't then do an undefined operation
in GCC itself.  This patch fixes the first case mentioned in the bug
report (I haven't been able to reproduce the second case, on trunk at
least).

	PR rtl-optimization/67483
	* combine.c (make_compound_operation): Don't call extract_left_shift
	with negative shift amounts.

From-SVN: r236232