[lld] [lld] Merge GOT entries for symbols that have been ICFed (PR #131630)

[Peter Smith via llvm-commits]

https://github.com/smithp35 commented:

Please could you write a full description for the commit message? There's a lot of subtle details about why this occur and why this is the best fix. 

It also helps people reading the commit log to not have to refer to an external URL for details.

I'm surprised the outliner has chosen to split apart an ADRP and a ldr. This prevents the relocation optimization
```
ADRP  x0, :got: symbol            // R_<CLS>_ADR_GOT_PAGE
LDR   x0, [x0 :got_lo12: symbol]  // R_<CLS>_LD64_GOT_LO12_NC

// after optimization:

ADRP  x0, symbol                  // R_<CLS>_ADR_PREL_PG_HI21
ADD   x0, x0, :lo12: symbol       // R_<CLS>_ADD_ABS_LO12_NC
```
Defined in https://github.com/ARM-software/abi-aa/blob/main/aaelf64/aaelf64.rst#579relocation-optimization 

It may be worth raising an issue to stop the outliner from doing that.

https://github.com/llvm/llvm-project/pull/131630