[RISCV] Implement adding a displacement to a BlockAddress

Recent change rL357393 uses MachineInstrBuilder::addDisp to add a based on a
BlockAddress but this case was not implemented.

This patch adds the missing case and a test for RISC-V that exercises the new
case.

Differential Revision: https://reviews.llvm.org/D60136

llvm-svn: 357752