Define AVX conversion intrinsics

This defines the following intrinsics:

 * `_mm256_cvtepi32_pd`
 * `_mm256_cvtepi32_ps`
 * `_mm256_cvtpd_epi32`
 * `_mm256_cvtpd_ps`
 * `_mm256_cvtps_epi32`
 * `_mm256_cvtps_pd`
 * `_mm256_cvttpd_epi32`
 * `_mm256_cvttps_epi32`

Intel reference: https://software.intel.com/en-us/node/514130.

Author: ruuda

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

@@ -15,6 +15,62 @@
             "ret": "f(32-64)",
             "args": ["s8SPc"]
         },
+        {
+            "intrinsic": "256_cvtepi32_pd",
+            "width": [256],
+            "llvm": "cvtdq2.pd.256",
+            "ret": "f64",
+            "args": ["s32h"]
+        },
+        {
+            "intrinsic": "256_cvtepi32_ps",
+            "width": [256],
+            "llvm": "cvtdq2.ps.256",
+            "ret": "f32",
+            "args": ["s32"]
+        },
+        {
+            "intrinsic": "256_cvtpd_epi32",
+            "width": [256],
+            "llvm": "cvt.pd2dq.256",
+            "ret": "s32h",
+            "args": ["f64"]
+        },
+        {
+            "intrinsic": "256_cvtpd_ps",
+            "width": [256],
+            "llvm": "cvt.pd2.ps.256",
+            "ret": "f32h",
+            "args": ["f64"]
+        },
+        {
+            "intrinsic": "256_cvtps_epi32",
+            "width": [256],
+            "llvm": "cvt.ps2dq.256",
+            "ret": "s32",
+            "args": ["f32"]
+        },
+        {
+            "intrinsic": "256_cvtps_pd",
+            "width": [256],
+            "llvm": "cvt.ps2.pd.256",
+            "ret": "f64",
+            "args": ["f32h"]
+        },
+        {
+            "intrinsic": "256_cvttpd_epi32",
+            "width": [256],
+            "llvm": "cvtt.pd2dq.256",
+            "ret": "s32h",
+            "args": ["f64"]
+        },
+        {
+            "intrinsic": "256_cvttps_epi32",
+            "width": [256],
+            "llvm": "cvtt.ps2dq.256",
+            "ret": "s32",
+            "args": ["f32"]
+        },
         {
             "intrinsic": "256_dp_ps",
             "width": [256],