[llvm] [PowerPC] Peephole address calculation in TOC memops (PR #76488)

[Sean Fertile via llvm-commits]

mandlebug wrote:

Something useful related to this came up in @stephenpeckham's presentation. Adding an offset to the large code model relocation expressions is not allowed, so I belive we couldn't transform something like


```
addis r3, x[TD]@u(2)
addi  r3, x[TD]@l(r3)
ld.      r5, 2(r3)
```
to
```
addis r3, x[TD]@u(2)
ld  r5, x[TD]@l + 2)(r3)
```

I though I'd comment here now so we won't forget to test this.

https://github.com/llvm/llvm-project/pull/76488