RISC-V: Handle no_insn in TARGET_SCHED_VARIABLE_ISSUE.

Reference: https://github.com/gcc-mirror/gcc/commit/d0bc0cb66bcb0e6a5a5a31a9e900e8ccc98e34e5



RISC-V should also be implemented to handle no_insn patterns for pipelining.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_sched_variable_issue): New function.
	(TARGET_SCHED_VARIABLE_ISSUE): New macro.

Co-authored-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Co-authored-by: Jeff Law <jlaw@ventanamicro.com>