i386: Use reference of struct ix86_frame to avoid copy

i386: Use reference of struct ix86_frame to avoid copy

When there is no need to make a copy of ix86_frame, we can use reference
of struct ix86_frame to avoid copy.

Tested on x86-64.

	* config/i386/i386.c (ix86_can_use_return_insn_p): Use reference
	of struct ix86_frame.
	(ix86_initial_elimination_offset): Likewise.
	(ix86_expand_split_stack_prologue): Likewise.

From-SVN: r254480