[RISCV] Don't emit LUI/ADDI MachineSDNodes from getAddr

Instead add RISCVISD opcodes that will be selected to LUI/ADDI
during isel.

I'm looking into maybe moving doPeepholeLoadStoreADDI into isel.
Having the ADDI as a RISCVISD node will make it visible to isel.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D127713