Introduce rtl_data::init_stack_alignment

Introduce rtl_data::init_stack_alignment

Move this part of "expand"'s initialization of crtl into its own
method so that it can used by the RTL frontend when postprocessing
RTL dumps.

gcc/ChangeLog:
	* cfgexpand.c (pass_expand::execute): Move stack initializations
	to rtl_data::init_stack_alignment and call it.
	* emit-rtl.c (rtl_data::init_stack_alignment): New method.
	* emit-rtl.h (rtl_data::init_stack_alignment): New method.

From-SVN: r243371