ldist: Further fixes for -ftrapv [PR94114]

ldist: Further fixes for -ftrapv [PR94114]

As the testcase shows, arithmetics that for -ftrapv would need multiple
basic blocks can show up not just in nb_bytes expressions where we
are calling rewrite_to_non_trapping_overflow for a while already,
but also in the pointer expression to the start of the region.
While the testcase covers just the first hunk and I've failed to create
a testcase for the latter, it is at least in theory possible too, so I've
adjusted that hunk too.

2020-03-11  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/94114
	* tree-loop-distribution.c (generate_memset_builtin): Call
	rewrite_to_non_trapping_overflow even on mem.
	(generate_memcpy_builtin): Call rewrite_to_non_trapping_overflow even
	on dest and src.

	* gcc.dg/pr94114.c: New test.

(cherry picked from commit 2fd27691f213f2e808626c4cd492b00c801a00fa)