[llvm] [GlobalISel][AArch64][AMDGPU] Lower FPOWI into series of multiplication (PR #95217)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

```
def expand_fpowi : GICombineRule<
   (defs root:$root, build_fn_matchinfo:$matchinfo),
   (match (G_CONSTANT $int, $imm)
          (G_FPOWI $root, $float, $int),
   [{ return Helper.matchExpandFPOWI(*${root}, ${matchinfo}); }]),
   (apply [{ Helper.applyBuildFnBO(*${root}, ${matchinfo}); }])>;
```

https://github.com/llvm/llvm-project/pull/95217