gimple: Ignore *0 = {CLOBBER} in path isolation [PR96722]

gimple: Ignore *0 = {CLOBBER} in path isolation [PR96722]

Clobbers of MEM_REF with NULL address are just fancy nops, something we just
ignore and don't emit any code for it (ditto for other clobbers), they just
mark end of life on something, so we shouldn't infer from those that there
is some UB.

2020-08-25  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/96722
	* gimple.c (infer_nonnull_range): Formatting fix.
	(infer_nonnull_range_by_dereference): Return false for clobber stmts.

	* g++.dg/opt/pr96722.C: New test.