[RISCV] Prevent use of t0(aka x5) as rs1 for jalr instructions.

Some microarchitectures treat rs1=x1/x5 on jalr as a hint to pop
the return-address stack. We should avoid using x5 on jalr
instructions since we aren't using x5 as an alternate link register.

Differential Revision: https://reviews.llvm.org/D105875