[llvm] ARM: Remove override of shouldRewriteCopySrc (PR #125219)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> From what I can tell if we peephole to these 'invalid' copies,

No optimization is allowed to introduce illegal copies. This is supposed to be an optimization hint, the target isn't supposed to be responsible for avoiding invalid code here.

> Do you have a reason to remove this? 

I'm pretty sure the existence of this is a hack (I added the hook in the first place), and not having purely generic logic may complicate another fix I'm working on.

> The case I came up with where this did change things it made them better by removing some copies via gprs.

Then that needs to be reflected in a testcase. If code can be deleted and no test change, it should be deleted 

https://github.com/llvm/llvm-project/pull/125219