i386: Use const reference of struct ix86_frame to avoid copy

i386: Use const reference of struct ix86_frame to avoid copy

We can use const reference of struct ix86_frame to avoid making a local
copy of ix86_frame.  ix86_expand_epilogue makes a local copy of struct
ix86_frame and uses the reg_save_offset field as a local variable.  This
patch uses a separate local variable for reg_save_offset.

Tested on x86-64 with ada.

	PR target/83905
	* config/i386/i386.c (ix86_expand_prologue): Use cost reference
	of struct ix86_frame.
	(ix86_expand_epilogue): Likewise.  Add a local variable for
	the reg_save_offset field in struct ix86_frame.

From-SVN: r256996