Add support for Vector Pack Pixel on PowerPC

The llvm intrinsic uses signed integers.

src/etc/platform-intrinsics/powerpc.json

@@ -135,6 +135,13 @@
             "llvm": "vpk{1.kind}{1.data_type_short}{0.kind}s",
             "ret": "u(8-16)",
             "args": ["0Ws", "1"]
+        },
+        {
+            "intrinsic": "packpx",
+            "width": [128],
+            "llvm": "vpkpx",
+            "ret": "s16",
+            "args": ["s32", "s32"]
         }
     ]
 }

src/librustc_platform_intrinsics/powerpc.rs

@@ -312,6 +312,11 @@ pub fn find(name: &str) -> Option<Intrinsic> {
             output: &::U16x8,
             definition: Named("llvm.ppc.altivec.vpkswus")
         },
+        "_vec_packpx" => Intrinsic {
+            inputs: { static INPUTS: [&'static Type; 2] = [&::I32x4, &::I32x4]; &INPUTS },
+            output: &::I16x8,
+            definition: Named("llvm.ppc.altivec.vpkpx")
+        },
         _ => return None,
     })
 }