[arm][aarch64] Handle no_insn in TARGET_SCHED_VARIABLE_ISSUE

[arm][aarch64] Handle no_insn in TARGET_SCHED_VARIABLE_ISSUE

Since no_insn patterns expand to no instructions, they shouldn't
count against the issue rate, just like USEs and CLOBBERs don't.

2019-09-17  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_sched_variable_issue): New
	function.
	(TARGET_SCHED_VARIABLE_ISSUE): New macro.
	* config/arm/arm.c (arm_sched_variable_issue): New function.
	(TARGET_SCHED_VARIABLE_ISSUE): New macro.

From-SVN: r275808