riscv64: Add a "GVN$after_arch" pass.
This makes 3 previously failing checker tests pass.
Test: testrunner.py --target --optimizing
# Ignore 3 pre-existing failures (137-cfi and 2x checker).
Bug: 283082089
Change-Id: Ie45a32fbdd5439e2abd123d83fc9992debae06f2