re PR target/53988 ([SH] tst Rm,Rn not used for QI/HImode)

re PR target/53988 ([SH] tst Rm,Rn not used for QI/HImode)

gcc/
	PR target/53988
	* config/sh/sh-protos.h (sh_find_set_of_reg): Make sure not to return
	nullptr for insn when reaching the first insn.
	* config/sh/sh.c (sh_unspec_insn_p): Rewrite using subrtx_iterator.
	(sh_insn_operands_modified_between_p): Add nullptr check.
	(sh_find_extending_set_of_reg): Fix log message.  Don't accept
	sign extending mem load if the insn contains any UNSPEC or
	UNSPEC_VOLATILE.

From-SVN: r219864