tree-optimization: Fix tree dse of strncpy PR93249

tree-optimization: Fix tree dse of strncpy PR93249

As the testcase shows, tail trimming of strncpy in tree-ssa-dse.c is fine,
we just copy or clear fewer bytes in the destination, but unlike
memcpy/memset etc., head trimming is problematic in certain cases.
If we can prove that there are no zero bytes among initial head_trim bytes,
it is ok to trim it, if we can prove there is at least one zero byte among
initial head_trim bytes, we could (not implemented in the patch) turn
the strncpy into memset 0, but otherwise we need to avoid the head trimming,
because the presence or absence of NUL byte there changes the behavior for
subsequent bytes, whether further bytes from src are copied or if further
bytes are cleared.

2020-01-15  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/93249
	* tree-ssa-dse.c: Include builtins.h and gimple-fold.h.
	(maybe_trim_memstar_call): Move head_trim and tail_trim vars to
	function body scope, reindent.  For BUILTIN_IN_STRNCPY*, don't
	perform head trim unless we can prove there are no '\0' chars
	from the source among the first head_trim chars.

	* gcc.c-torture/execute/pr93249.c: New test.