[llvm] [PowerPC] Let base implementation decide if MI is rematerizable by default (PR #75772)

[Nemanja Ivanovic via llvm-commits]

https://github.com/nemanjai approved this pull request.

I believe that the thinking was that this would help catch an issue where we set the Remat flag on an instruction and forget to update this. But the requirement is too strong since this may be called with target independent instructions.
Perhaps we can get both by asserting that the instruction has a size of zero (or at least less than 4) in the `default` case.
In any case, this unblocks a bot/commit, so LGTM.

https://github.com/llvm/llvm-project/pull/75772