[RISCV] Fix corner cases after #170070 (#170438)
There are two fixes:
1. Clear kill flags for `FalseReg` in foldVMergeToMask or we can't
pass the MachineVerifier because of using a killed virtual register.
2. Restrict `lookThruCopies` to only look through COPYs with
one non-debug use.
This was found when backporting #170070 to 21.x branch.
