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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> The best test case I came up with was https://godbolt.org/z/4vGKaTr4s, which for this patch produced one of those copies, but it was later relaxed into two.

I see this did change, but the result looks better? It's fewer real instructions. I added the test as a baseline. I can submit that separately if the old result was really better.

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