RISC-V: Disable user vsetvl fusion into EMPTY or DIRTY (Polluted EMPTY) block

RISC-V: Disable user vsetvl fusion into EMPTY or DIRTY (Polluted EMPTY) block

This patch is fixing these bunch of ICE in "vect" testsuite:
FAIL: gcc.dg/vect/no-scevccp-outer-2.c (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/no-scevccp-outer-2.c (test for excess errors)
FAIL: gcc.dg/vect/pr109025.c (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/pr109025.c (test for excess errors)
FAIL: gcc.dg/vect/pr109025.c -flto -ffat-lto-objects (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/pr109025.c -flto -ffat-lto-objects (test for excess errors)
FAIL: gcc.dg/vect/pr42604.c (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/pr42604.c (test for excess errors)
FAIL: gcc.dg/vect/pr42604.c -flto -ffat-lto-objects (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/pr42604.c -flto -ffat-lto-objects (test for excess errors)
FAIL: gcc.dg/vect/vect-double-reduc-3.c (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/vect-double-reduc-3.c (test for excess errors)
FAIL: gcc.dg/vect/vect-double-reduc-3.c -flto -ffat-lto-objects (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/vect-double-reduc-3.c -flto -ffat-lto-objects (test for excess errors)
FAIL: gcc.dg/vect/vect-double-reduc-7.c (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/vect-double-reduc-7.c (test for excess errors)
FAIL: gcc.dg/vect/vect-double-reduc-7.c -flto -ffat-lto-objects (internal compiler error: in anticipatable_occurrence_p, at config/riscv/riscv-vsetvl.cc:314)
FAIL: gcc.dg/vect/vect-double-reduc-7.c -flto -ffat-lto-objects (test for excess errors)

gcc/ChangeLog:

	* config/riscv/riscv-vsetvl.cc (pass_vsetvl::earliest_fusion): Fix bug.