[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? I tried that over in [#208341 (comment)](https://github.com/llvm/llvm-project/pull/208341#issuecomment-4919997448) but got these errors on AMDGPU
> 
> ```
> # | error: no libcall available for fldexp
> # | error: <unknown>:0:0: in function s_exp10_f32 void (ptr addrspace(1), float): unsupported call to function <unknown>
> # | 
> # | error: no libcall available for fldexp
> # | error: <unknown>:0:0: in function s_exp10_v2f32 void (ptr addrspace(1), <2 x float>): unsupported call to function <unknown>
> ```


This doesn't make any sense. AMDGPU has no libcalls, and has ldexp as a native instruction 



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