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

[David Green via llvm-commits]

davemgreen wrote:

> I have some stuck patches that introduce more subregister uses that are breaking all of the backends

I wouldn't have expected to see subregs on most instructions prior to, say, the Register Coalescer.

I think this function can be removed.

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