mmix.md ("return"): New pattern.

	* config/mmix/mmix.md ("return"): New pattern.
	* config/mmix/mmix.h (TARGET_MASK_USE_RETURN_INSN)
	(TARGET_USE_RETURN_INSN): New macros.
	(TARGET_DEFAULT): Include TARGET_MASK_USE_RETURN_INSN.
	(TARGET_SWITCHES): Add -msingle-exit and -mno-single-exit.
	* config/mmix/mmix.c (MMIX_OUTPUT_REGNO): Fix spacing.
	(MMIX_POP_ARGUMENT): New macro.
	(mmix_target_asm_function_prologue): When no epilogue is executed,
	just emit a blank line.  Use MMIX_POP_ARGUMENT with final POP insn.
	(mmix_print_operand) <case '.'>: New case.
	(mmix_print_operand_punct_valid_p): Match '.'.
	(mmix_use_simple_return): New function.
	* config/mmix/mmix-protos.h (mmix_use_simple_return): Prototype.
	* doc/invoke.texi (Option Summary) <MMIX Summary>: Add
	-msingle-exit, -mno-single-exit.
	(MMIX Options): Ditto.

From-SVN: r55121