tree-optimization/100278 - handle mismatched code in TBAA adjust of PRE

tree-optimization/100278 - handle mismatched code in TBAA adjust of PRE

PRE has code to adjust TBAA behavior for refs that expects the base
operation code to match.  The testcase shows a case where we have
a VAR_DECL vs. a MEM_REF so add code to give up in such cases.

2021-04-27  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100278
	* tree-ssa-pre.c (compute_avail): Give up when we cannot
	adjust TBAA beacuse of mismatching bases.

	* gcc.dg/tree-ssa/pr100278.c: New testcase.