[RISCV] Support lowering FrameIndex

Introduces the AddrFI "addressing mode", which is necessary simply because 
it's not possible to write a pattern that directly matches a frameindex.

Ensure callee-saved registers are accessed relative to the stackpointer. This
is necessary as callee-saved register spills are performed before the frame
pointer is set.

Move HexagonDAGToDAGISel::isOrEquivalentToAdd to SelectionDAGISel, so we can 
make use of it in the RISC-V backend.

Differential Revision: https://reviews.llvm.org/D39848

llvm-svn: 320353