[llvm] RegisterCoalescer: Remove empty subranges after merging subranges (PR #222050)

[Carl Ritson via llvm-commits]

https://github.com/perlfu commented:

Code change seems fine to me.

I can't really speak to the ARM assembly in the test, but I wonder if the test should really be checking the output.  Presumably this is just a "doesn't assert" test?

https://github.com/llvm/llvm-project/pull/222050