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

[David Green via llvm-commits]

davemgreen wrote:

Yep I agree - presumably the tests for this problem are no longer hitting the same issue due to other changes.

As far as I understand the issue - Arm has d registers that overlap with s registers, so s0+s1 == d0 and s2+s3 == d1 etc.  There are 32 s registers, which alias the first 16 d registers. But (with the right features) there are 32 d registers, the top/bottom half of which have no corresponding s regs, so a dpr + subreg -> reg copy is not valid if an upper d reg is used.

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.

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