[RISCV] Share code for fixed offsets adjustRegs (thus materializing fewer constants)

This reuses the existing optimized implementation of adjustReg, and commons up code. This has the effect of enabling two code changes for the new caller. First, we enable the "split andi" lowering (with no alignment requirement), and second we use a sub with smaller constant in register instead of a add with negative constant in register.

Differential Revision: https://reviews.llvm.org/D132839