[llvm] [PowerPC] Support conversion between f16 and f128 (PR #130158)

[via llvm-commits]

dyung wrote:

@lei137 The cherry-pick of this commit to the `release/20.x` branch seems to be seeing a failure in the test `llvm/test/CodeGen/PowerPC/f128-conv.ll` on the following bots, can you take a look?
 - https://lab.llvm.org/buildbot/#/builders/115/builds/86
 - https://lab.llvm.org/buildbot/#/builders/62/builds/120
 - https://lab.llvm.org/buildbot/#/builders/156/builds/71
 - https://lab.llvm.org/buildbot/#/builders/31/builds/113
 - https://lab.llvm.org/buildbot/#/builders/100/builds/133
 - https://lab.llvm.org/buildbot/#/builders/102/builds/163

https://github.com/llvm/llvm-project/pull/130158