Terminate BB analysis on NULL memory access in ipa-pure-const and ipa-modref

Terminate BB analysis on NULL memory access in ipa-pure-const and ipa-modref

As discussed in the PR, we miss some optimization becuase
gimple-ssa-isolate-paths turns NULL memory accesses to volatile and adds
__builtin_trap after them.  This is seen as a side-effect by IPA analysis
and additionally the (fully unreachable) builtin_trap is believed to load
all global memory.

I think we should think of less intrusive gimple representation of this, but
it is also easy enough to special case that in IPA analysers as done in
this patch.  This is a win even if we improve the representation since
gimple-ssa-isolate-paths is run late and this way we improve optimization
early.

This affects 1623 functions during cc1plus link.

Bootstrapped/regtested x86_64-linux, comitted.

gcc/ChangeLog:

2021-12-12  Jan Hubicka  <hubicka@ucw.cz>

	PR ipa/103665
	* ipa-modref.c (modref_access_analysis::analyze): Terminate BB
	analysis on NULL memory access.
	* ipa-pure-const.c (analyze_function): Likewise.