[RISCV] Implement evaluateBranch

This implements the instruction analysis required to print branch
targets as part of llvm-objdump's disassembly.

Note, this only handles those branches which can be analyzed in a single
instruction, a future patch will handle multiple-instruction patterns,
such as AUIPC/LUI+JALR instruction pairs.

Differential Revision: https://reviews.llvm.org/D77567