[RISCV][WIP] Load/store relaxation

[gkm]

Summary

Relax loads, stores, and address arithmetic for absolute, pc-relative, gp-relative
and tp-relative addressing.

[ Caveat: this diff still has some gaps in implementation and testing. ]

Depends on D125497

[gkm]

• fix indentation, as caught by clang-format

[gkm]

• rebase
• rework HiLoAddend as a union