[compiler-rt] [compiler-rt][ARM] Optimized integer -> FP conversions (PR #179928)

[Simon Tatham via llvm-commits]

statham-arm wrote:

Hi @compnerd, this is the next patch in my optimized FP series. (Second to last!)

I think I've addressed the usual nits this time: `#(a << b)`, misaligned comments and reversing `#if !__thumb__`.

https://github.com/llvm/llvm-project/pull/179928