[PPC] Implement vsubcuw

Other half of #2125. I don't know of any title that utilizes this instruction, but I went ahead and implemented it for completeness.

Verified the implementation with `instr__gen_vsubcuw` from #1348. Can be grabbed with:
```
git checkout origin/gen_tests -- src\xenia\cpu\ppc\testing\*vsubcuw.s
```

Author: Wunkolo

src/xenia/cpu/ppc/ppc_emit_altivec.cc

@@ -1662,7 +1662,11 @@ int InstrEmit_vsrw128(PPCHIRBuilder& f, const InstrData& i) {
 }
 
 int InstrEmit_vsubcuw(PPCHIRBuilder& f, const InstrData& i) {
-  XEINSTRNOTIMPLEMENTED();
+  Value* underflow =
+      f.VectorCompareUGE(f.LoadVR(i.VX.VA), f.LoadVR(i.VX.VB), INT32_TYPE);
+  Value* borrow =
+      f.VectorShr(underflow, f.LoadConstantVec128(vec128i(31)), INT32_TYPE);
+  f.StoreVR(i.VX.VD, borrow);
   return 1;
 }