[llvm] [ARM][Thumb1] Improve EstimateFunctionSizeInBytes accuracy (PR #203318)

[Peter Smith via llvm-commits]

https://github.com/smithp35 commented:

I think this implementation is in line with the RFC. I'm not in a good position to double check the constants, or think of anything you may have missed.

https://github.com/llvm/llvm-project/pull/203318