RISC-V: Add optim-no-fusion compile option [VSETVL PASS]

RISC-V: Add optim-no-fusion compile option [VSETVL PASS]

This patch adds no fusion compile option to disable phase 2 global fusion.

It can help us to analyze the compile-time and debugging.

Committed.

gcc/ChangeLog:

	* config/riscv/riscv-opts.h (enum vsetvl_strategy_enum): Add optim-no-fusion option.
	* config/riscv/riscv-vsetvl.cc (pass_vsetvl::lazy_vsetvl): Ditto.
	(pass_vsetvl::execute): Ditto.
	* config/riscv/riscv.opt: Ditto.