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

[Matt Arsenault via llvm-commits]

arsenm wrote:

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

Some selections directly emit them, plus not folding subreg uses in blocks CSE and a few other optimizations. Plus convertToThreeAddress is called during the coalescer, and in greedy.

> I think this function can be removed.

Don't know, but I'm not going to attempt it. Merge this or not then?

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