Define x86 fused multiply-add intrinsics

This defines the following intrinsics for 128 and 256 bit vectors of f32
and f64:

 * `fmadd`
 * `fmaddsub`
 * `fmsub`
 * `fmsubadd`
 * `fnmadd`
 * `fnmsub`

The `_sd` and `_ss` variants are not included yet.

Intel intrinsic reference: https://software.intel.com/en-us/node/523929

The intrinsics there are listed under AVX2, but in the Intel Intrinsic
Guide they are part of the "FMA" technology, and LLVM puts them under
FMA, not AVX2.

Author: ruuda

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

@@ -0,0 +1,47 @@
+{
+    "llvm_prefix": "llvm.x86.fma.",
+    "intrinsics": [
+        {
+            "intrinsic": "{0.width_mm}_fmadd_{0.data_type}",
+            "width": [128, 256],
+            "llvm": "vfmadd.{0.data_type_short}{0.width_suffix}",
+            "ret": "f(32-64)",
+            "args": ["0", "0", "0"]
+        },
+        {
+            "intrinsic": "{0.width_mm}_fmaddsub_{0.data_type}",
+            "width": [128, 256],
+            "llvm": "vfmaddsub.{0.data_type_short}{0.width_suffix}",
+            "ret": "f(32-64)",
+            "args": ["0", "0", "0"]
+        },
+        {
+            "intrinsic": "{0.width_mm}_fmsub_{0.data_type}",
+            "width": [128, 256],
+            "llvm": "vfmsub.{0.data_type_short}{0.width_suffix}",
+            "ret": "f(32-64)",
+            "args": ["0", "0", "0"]
+        },
+        {
+            "intrinsic": "{0.width_mm}_fmsubadd_{0.data_type}",
+            "width": [128, 256],
+            "llvm": "vfmsubadd.{0.data_type_short}{0.width_suffix}",
+            "ret": "f(32-64)",
+            "args": ["0", "0", "0"]
+        },
+        {
+            "intrinsic": "{0.width_mm}_fnmadd_{0.data_type}",
+            "width": [128, 256],
+            "llvm": "vfnmadd.{0.data_type_short}{0.width_suffix}",
+            "ret": "f(32-64)",
+            "args": ["0", "0", "0"]
+        },
+        {
+            "intrinsic": "{0.width_mm}_fnmsub_{0.data_type}",
+            "width": [128, 256],
+            "llvm": "vfnmsub.{0.data_type_short}{0.width_suffix}",
+            "ret": "f(32-64)",
+            "args": ["0", "0", "0"]
+        }
+    ]
+}