tree-optimization/109491 - ICE in expressions_equal_p

tree-optimization/109491 - ICE in expressions_equal_p

At some point I elided the NULL pointer check in expressions_equal_p
because it shouldn't be necessary not realizing that for example
TARGET_MEM_REF has optional operands we cannot substitute with
something non-NULL with the same semantics.  The following does the
simple thing and restore the check removed in r11-4982.

	PR tree-optimization/109491
	* tree-ssa-sccvn.c (expressions_equal_p): Restore the
	NULL operands test.

(cherry picked from commit a37783de23c067d6a26374ff29c014e49604035c)