[RISCV] Fold ADDIs into load/stores with nonzero offsets

We can often fold an ADDI into the offset of load/store instructions:

   (load (addi base, off1), off2) -> (load base, off1+off2)
   (store val, (addi base, off1), off2) -> (store val, base, off1+off2)

This is possible when the off1+off2 continues to fit the 12-bit immediate.
We remove the previous restriction where we would never fold the ADDIs if
the load/stores had nonzero offsets. We now do the fold the the resulting
constant still fits a 12-bit immediate, or if off1 is a variable's address
and we know based on that variable's alignment that off1+offs2 won't overflow.

Differential Revision: https://reviews.llvm.org/D79690