[llvm] [ARM] Remove duplicate custom SDag node (NFCI) (PR #93419)

[David Green via llvm-commits]

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

SUBS might be a better name given the final instruction, but considering the other nodes that exist (ADDE/ADDC) then this makes sense.

There is a difference between the glue of the two nodes, but as far as I understand this should be OK. LGTM, thanks for the cleanup.

https://github.com/llvm/llvm-project/pull/93419