Ensure partial Vector256 acceleration is still possible on AVX only hardware (#73720)

* Ensure that Vector256 APIs exposed in .NET Core 3.1 remain accelerated in .NET 7

* Ensure new Vector256 APIs that are trivially limited to Avx only are accelerated

Author: tannergooding

src/coreclr/jit/hwintrinsic.cpp

@@ -383,8 +383,8 @@ NamedIntrinsic HWIntrinsicInfo::lookupId(Compiler*         comp,
             NamedIntrinsic ni = intrinsicInfo.id;
 
 #if defined(TARGET_XARCH)
-            // on AVX1-only CPUs we only support NI_Vector256_Create intrinsic in Vector256
-            if (isLimitedVector256Isa && (ni != NI_Vector256_Create))
+            // on AVX1-only CPUs we only support a subset of intrinsics in Vector256
+            if (isLimitedVector256Isa && !AvxOnlyCompatible(ni))
             {
                 return NI_Illegal;
             }

src/coreclr/jit/hwintrinsic.h

@@ -147,7 +147,11 @@ enum HWIntrinsicFlag : unsigned int
     // Returns Per-Element Mask
     // the intrinsic returns a vector containing elements that are either "all bits set" or "all bits clear"
     // this output can be used as a per-element mask
-    HW_Flag_ReturnsPerElementMask = 0x20000
+    HW_Flag_ReturnsPerElementMask = 0x20000,
+
+    // AvxOnlyCompatible
+    // the intrinsic can be used on hardware with AVX but not AVX2 support
+    HW_Flag_AvxOnlyCompatible = 0x40000,
 
 #elif defined(TARGET_ARM64)
     // The intrinsic has an immediate operand
@@ -658,6 +662,14 @@ struct HWIntrinsicInfo
 #endif
     }
 
+#if defined(TARGET_XARCH)
+    static bool AvxOnlyCompatible(NamedIntrinsic id)
+    {
+        HWIntrinsicFlag flags = lookupFlags(id);
+        return (flags & HW_Flag_AvxOnlyCompatible) != 0;
+    }
+#endif
+
     static bool BaseTypeFromFirstArg(NamedIntrinsic id)
     {
         HWIntrinsicFlag flags = lookupFlags(id);