xtensa: Simplify several MD templates

xtensa: Simplify several MD templates

No functional changes.

gcc/ChangeLog:

	* config/xtensa/predicates.md
	(subreg_HQI_lowpart_operator, xtensa_sminmax_operator):
	New operator predicates.
	* config/xtensa/xtensa-protos.h (xtensa_match_CLAMPS_imms_p):
	Remove.
	* config/xtensa/xtensa.cc (xtensa_match_CLAMPS_imms_p): Ditto.
	* config/xtensa/xtensa.md
	(*addsubx, *extzvsi-1bit_ashlsi3, *extzvsi-1bit_addsubx):
	Revise the output statements by conditional ternary operator rather
	than switch-case clause in order to avoid using gcc_unreachable().
	(xtensa_clamps): Reduce to a single pattern definition using the
	predicate added above.
	(Some split patterns to assist *masktrue_const_bitcmpl): Ditto.