alias.c:find_base_term and find_base_value checked:

      if (GET_MODE_SIZE (GET_MODE (src)) < GET_MODE_SIZE (Pmode))

but (a) comparing the precision seems more correct, since it's possible
for modes to have the same memory size as Pmode but fewer bits and
(b) the functions are called on arbitrary rtl, so there's no guarantee
that we're handling an integer truncation.

Since there's no point processing truncations of anything other than an
integer, this patch checks that first.

2017-11-01  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* alias.c (find_base_value, find_base_term): Only process integer
	truncations.  Check the precision rather than the size.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r254306