This fixes an integer overflow warning that ultimatively happens because
of TREE_OVERFLOW propagating through transforms and the existing guard
against this,

375           if (TREE_OVERFLOW_P (ret)
376               && !TREE_OVERFLOW_P (op0)
377               && !TREE_OVERFLOW_P (op1))
378             overflow_warning (EXPR_LOC_OR_LOC (expr, input_location,

being insufficient.  Rather than trying to use sth like walk_tree to
exhaustively walk operands (with the possibility of introducing
quadraticness when folding larger expressions recursively) the
following amends the above with an ad-hoc test for a binary op0
with a possibly constant op1.

2020-05-30  Richard Biener  <rguenther@suse.de>

	PR c/95141
gcc/c
	* c-fold.c (c_fully_fold_internal): Enhance guard on
	overflow_warning.

gcc/testsuite
	* gcc.dg/pr95141.c: New testcase.