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

[Simon Tatham via llvm-commits]

statham-arm wrote:

... ok, the CI failure pointed out that I'd foolishly only run the tests in `llvm/test/CodeGen/Thumb`, but some tests in `.../ARM` are now failing to verify. Looks as if the failing tests involve CMSE and/or KCFI, which add huge things to the prologue which I didn't account for. But it's late in the day and hot, so I'll fix that tomorrow!

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