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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-expensive-checks-debian` running on `gribozavr4` while building `llvm` at step 6 "test-build-unified-tree-check-all".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/16/builds/18417
> 
> Here is the relevant piece of the build log for the reference

> Register class DPR does not fully support subreg index ssub_0

Unrepresentable cases should have been filtered out by the default logic 



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