RISC-V: Fix ICE of visiting non-existing block in CFG.

RISC-V: Fix ICE of visiting non-existing block in CFG.

This patch is to fix issue of visiting non-existing block of CFG.
Since blocks index of CFG in GCC are not always contiguous, we will potentially
visit a gap block which is no existing in the current CFG.

This patch can avoid visiting non existing block in CFG.

I noticed such issue in my internal regression of current testsuite
when I change the X86 server machine. This patch fix it:
17:27:15      job(build_and_test_rv32): Increased FAIL List:
17:27:15      job(build_and_test_rv32): FAIL: gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-46.c
-O2 -flto -fno-use-linker-plugin -flto-partition=none  (internal compiler error: Segmentation fault)

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc
	(pass_vsetvl::compute_global_backward_infos): Change to visit CFG.
	(pass_vsetvl::prune_expressions): Ditto.