[llvm] [SystemZ] Remove custom implementation of optimizeLoadInst [NFC] (PR #133123)

[Ulrich Weigand via llvm-commits]

uweigand wrote:

> I wasn't 100% given the difference in sub-register, multiple use and defs, but from the SystemZ code, it looks like those cases simply don't arise?

Folding sub-register loads was not done by the SystemZ code, but adding that support should be beneficial, I think.

I *do* wonder about multiple uses - if a loaded value is used in multiple instructions, and some of those instructions are changed to fold in the load, this will increase the total number of memory accesses.  That might not be a big deal (value will likely be in L1), but it's not immediately clear either how this is beneficial ...   (However, folding to multiple uses within the same instruction likely is beneficial when possible, if there are no further uses outside that single instruction.)

In any case, this is not a platform-specific question, so I'm fine with the custom implementation going away.

https://github.com/llvm/llvm-project/pull/133123