riscv64: Fix wrong sign-extension for references.

Test: Modify kPreferredAllocSpaceBegin = 0x90000000, then
      testrunner.py --target --64 --ndebug --optimizing
Bug: 283082089
Change-Id: Ifb82d616a0d9664a2e7f5f96a1a79ddce5862cdf
7 files changed