(struct clear_by_pieces): New structure.

(clear_by_pieces{,_1}, {is,mostly}_zeros_p): New functions.
(clrstr_optab): New optab.
(clear_storage): Rework to try to use clear_by_pieces, then
new clrstr insn, then library call.
(store_constructor): Track if target is already cleared.
Clear target first if CONSTRUCTOR is mostly zeros.
Don't write zeros if target has been cleared.
Add new arg to clear_storage call.
(expand_expr, case CONSTRUCTOR): Don't put static constructor
in memory if mostly zero.

From-SVN: r10842