analyzer: fix NULL deref false positives [PR94851]

analyzer: fix NULL deref false positives [PR94851]

PR analyzer/94851 reports various false "NULL dereference" diagnostics.
The first case (comment #1) affects GCC 10.2 but no longer affects
trunk; I believe it was fixed by the state rewrite of
r11-2694-g808f4dfeb3a95f50f15e71148e5c1067f90a126d.

The patch adds a regression test for this case.

The other cases (comment #3 and comment #4) still affect trunk.
In both cases, the && in a conditional is optimized to bitwise &
  _1 = p_4 != 0B;
  _2 = p_4 != q_6(D);
  _3 = _1 & _2;
and the analyzer fails to fold this for the case where one (or both) of
the conditionals is false, and thus erroneously considers the path where
"p" is non-NULL despite being passed a NULL value.

Fix this by implementing folding for this case.

gcc/analyzer/ChangeLog:
	PR analyzer/94851
	* region-model-manager.cc
	(region_model_manager::maybe_fold_binop): Fold bitwise "& 0" to 0.

gcc/testsuite/ChangeLog:
	PR analyzer/94851
	* gcc.dg/analyzer/pr94851-1.c: New test.
	* gcc.dg/analyzer/pr94851-3.c: New test.
	* gcc.dg/analyzer/pr94851-4.c: New test.