[patch, fortran] Add string padding on assignment

[Thomas Koenig]

Hello world,

this patch addresses the case where a constant string is assigned to a
longer string, requiring padding with blanks.  This adds the blanks in
the front end, transforming

character*2 a
a = 'y'


into
character*2 a
a = 'y '

To avoid bloat for strings, I set an arbitrary limit of <8 spaces to be
padded, but of course I'd welcome better suggesionts.

Regression-tested.  OK for trunk with or without a better strategy?

	Thomas

2010-09-27  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR fortran/45636
	* frontend-passes.c (optimize_assignment):  In an assignment,
	fill up blanks on the right-hand side if fewer than
	STRING_PAD_LIMIT characters are missing.

2010-09-27  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR fortran/45636
	* gfortran.dg/character_pad_1.f90:  New test.

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