Fix handling of unknown sizes in rtx_addr_can_trap_p

If the size passed in to rtx_addr_can_trap_p was zero, the frame
handling would get the size from the mode instead.  However, this
too can be zero if the mode is BLKmode, i.e. if we have a BLKmode
memory reference with no MEM_SIZE (which should be rare these days).
This meant that the conditions for a 4-byte access at offset X were
stricter than those for an access of unknown size at offset X.

This patch checks whether the size is still zero, as the
SYMBOL_REF handling does.

gcc/
2016-11-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* rtlanal.c (rtx_addr_can_trap_p_1): Handle unknown sizes.

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

From-SVN: r242476