[llvm] ARM: Fix losing subregisters in convertToThreeAddress (PR #141662)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Did you find this by inspection, or did you see it get used somewhere? I think we can remove it if it is not used anywhere and disabled by default.

I thought I saw a verifier error from this, but I did this patch months ago. I'm looking for all of these cases though, I have some stuck patches that introduce more subregister uses that are breaking all of the backends

https://github.com/llvm/llvm-project/pull/141662