[llvm] add `frexp`/`ldexp` expansion in `ExpandIrInsts` (PR #208556)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Now, can we get rid of the LegalizeDag implementation?

Other legalizations can plausibly want to emit ldexp or frexp, so I'm not sure we should do that. 

> The payoff is that we now get a working frexp and ldexp for f128 on windows.

I'm not sure why doing this in the IR helps with this situation. The benefit of the IR expansion is you can introduce the control flow, but I don't follow how this fixes this case 



https://github.com/llvm/llvm-project/pull/208556