i386: Clear stack protector scratch with zero/sign-extend instruction

i386: Clear stack protector scratch with zero/sign-extend instruction

Use unrelated register initializations using zero/sign-extend instructions
to clear stack protector scratch register.

Hanlde only SI -> DImode extensions for 64-bit targets, as this is the
only extension that triggers the peephole in a non-negligible number.

Also use explicit check for word_mode instead of mode iterator in peephole2
patterns to avoid pattern explosion.

gcc/ChangeLog:

	* config/i386/i386.md (stack_protect_set_1 peephole2):
	Explicitly check operand 2 for word_mode.
	(stack_protect_set_1 peephole2 #2): Ditto.
	(stack_protect_set_2 peephole2): Ditto.
	(stack_protect_set_3 peephole2): Ditto.
	(*stack_protect_set_4z_<mode>_di): New insn patter.
	(*stack_protect_set_4s_<mode>_di): Ditto.
	(stack_protect_set_4 peephole2): New peephole2 pattern to
	substitute stack protector scratch register clear with unrelated
	register initialization involving zero/sign-extend instruction.