i386: Update preferred stack boundary for leaf functions

i386: Update preferred stack boundary for leaf functions

preferred_stack_boundary may not be the minimum stack alignment.  For
leaf functions without TLS access, max_used_stack_slot_alignment may be
smaller.  We should update preferred_stack_boundary for leaf functions.

gcc/

	PR target/82166
	* config/i386/i386.c (ix86_finalize_stack_frame_flags): Properly
	compute the minimum stack alignment.  Also update preferred stack
	boundary for leaf functions.

gcc/testsuite/

	PR target/82166
	* gcc.target/i386/pr82166.c: New test.

From-SVN: r252895