[PowerPC][NFC] Cleanup load/store spilling code

Summary: Cleanup and commonize code used for spilling to the stack.

Reviewers: stefanp, nemanjai, #powerpc, kamaub

Reviewed By: nemanjai, #powerpc, kamaub

Subscribers: kamaub, hiraditya, wuzish, shchenz, llvm-commits, kbarton

Tags: #llvm, #powerpc

Differential Revision: https://reviews.llvm.org/D79736