[RISCV] Assume no-op addrspacecasts by default

To support OpenCL, which typically uses SPIR as an IR, non-zero address
spaces must be accounted for. This patch makes the RISC-V target assume
no-op address space casts across the board, which effectively removes
the need to support addrspacecast instructions in the backend.

For a RISC-V implementation with different configurations or specialized
address spaces where casts aren't no-ops, the function can be adjusted
as required.

Reviewed By: jrtc27

Differential Revision: https://reviews.llvm.org/D93536