Add leafy mode for zero-call-used-regs

Add leafy mode for zero-call-used-regs

Introduce 'leafy' to auto-select between 'used' and 'all' for leaf and
nonleaf functions, respectively.

for  gcc/ChangeLog

	* doc/extend.texi (zero-call-used-regs): Document leafy and
	variants thereof.
	* flag-types.h (zero_regs_flags): Add LEAFY_MODE, as well as
	LEAFY and variants.
	* function.cc (gen_call_ued_regs_seq): Set only_used for leaf
	functions in leafy mode.
	* opts.cc (zero_call_used_regs_opts): Add leafy and variants.

for  gcc/testsuite/ChangeLog

	* c-c++-common/zero-scratch-regs-leafy-1.c: New.
	* c-c++-common/zero-scratch-regs-leafy-2.c: New.
	* gcc.target/i386/zero-scratch-regs-leafy-1.c: New.
	* gcc.target/i386/zero-scratch-regs-leafy-2.c: New.