[llvm] [PowerPC] Support conversion between f16 and f128. (PR #97677)

[Trevor Gross via llvm-commits]

tgross35 wrote:

Should tests at [`f128-conv.ll`](https://github.com/llvm/llvm-project/blob/ad82d1c53f089937c05af11ff45798ceb5ca894e/llvm/test/CodeGen/PowerPC/f128-conv.ll) or [`fp128-libcalls.ll`](https://github.com/llvm/llvm-project/blob/ad82d1c53f089937c05af11ff45798ceb5ca894e/llvm/test/CodeGen/PowerPC/fp128-libcalls.ll) be updated?

https://github.com/llvm/llvm-project/pull/97677