Define AVX comparison intrinsics

This defines `_mm256_cmp_pd` and `_mm256_cmp_ps`.

Intel reference: https://software.intel.com/en-us/node/524075.

src/etc/platform-intrinsics/x86/avx.json

@@ -15,6 +15,13 @@
             "ret": "f(32-64)",
             "args": ["s8SPc"]
         },
+        {
+            "intrinsic": "256_cmp_{0.data_type}",
+            "width": [256],
+            "llvm": "cmp.{1.data_type}.256",
+            "ret": "f(32-64)",
+            "args": ["0", "0", "s8S"]
+        },
         {
             "intrinsic": "256_cvtepi32_pd",
             "width": [256],