PR 78534 Reinstate better string copy algorithm

PR 78534 Reinstate better string copy algorithm

As part of the change to larger character lengths, the string copy
algorithm was temporarily pessimized to get around some spurious
-Wstringop-overflow warnings.  Having tried a number of variations of
this algorithm I have managed to get it down to one spurious warning,
only with -O1 optimization, in the testsuite.  This patch reinstates
the optimized variant and modifies this one testcase to ignore the
warning.

Regtested on x86_64-pc-linux-gnu.

gcc/fortran/ChangeLog:

2018-01-31  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/78534
	* trans-expr.c (fill_with_spaces): Use memset instead of
	generating loop.
	(gfc_trans_string_copy): Improve opportunity to use builtins with
	constant lengths.

gcc/testsuite/ChangeLog:

2018-01-31  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/78534
	* gfortran.dg/allocate_deferred_char_scalar_1.f03: Prune
	-Wstringop-overflow warnings due to spurious warning with -O1.
	* gfortran.dg/char_cast_1.f90: Update dump scan pattern.
	* gfortran.dg/transfer_intrinsic_1.f90: Likewise.

From-SVN: r257233