[RISCV] Reduce scalar load/store isel patterns to a single ComplexPattern. NFCI

Previously we had 3 different isel patterns for every scalar load
store instruction.

This reduces them to a single ComplexPattern that returns the Base
and Offset. Or an offset of 0 if there was no offset identified

I've done a similar thing for the 2 isel patterns that match add/or
with FrameIndex and immediate. Using the offset of 0, I was also
able to remove the custom handler for FrameIndex. Happy to split that
to another patch.

We might be able to enhance in the future to remove the post-isel
peephole or the special handling for ADD with constant added by D126576.

A nice side effect is that this removes nearly 3000 bytes from the isel
table.

Differential Revision: https://reviews.llvm.org/D126932