[PowerPC] Change half to use soft promotion rather than PromoteFloat

On PowerPC targets, `half` uses the default legalization of promoting to
a `f32`. However, this has some fundamental issues related to inability
to round trip. Resolve this by switching to the soft legalization, which
passes `f16` as an `i16`.

The PowerPC ABI Specification does not define a `_Float16` type, so the
calling convention changes are acceptable.

Fixes the PowerPC portion of [1]. A similar change was done for MIPS [2]
and Loongarch [3].

[1]: llvm#97975
[2]: llvm#110199
[3]: llvm#107791

Author: tgross35

llvm/docs/ReleaseNotes.md

@@ -101,6 +101,8 @@ Changes to the MIPS Backend
 Changes to the PowerPC Backend
 ------------------------------
 
+* `half` now uses a soft float ABI, which works correctly in more cases.
+
 Changes to the RISC-V Backend
 -----------------------------
 

llvm/lib/Target/PowerPC/PPCISelLowering.h

@@ -801,6 +801,8 @@ namespace llvm {
 
     bool useSoftFloat() const override;
 
+    bool softPromoteHalfType() const override { return true; }
+
     bool hasSPE() const;
 
     MVT getScalarShiftAmountTy(const DataLayout &, EVT) const override {