Don't notify crossgen2 about Vector<T> size when not needed (#34614)

- Stop notifying  about usage status of AVX2 when using isSubRegisterSIMDType
- This allows use of the Vector2/3/4 simd types in crossgen'd code without fixing whether or not Avx2 is required

src/coreclr/src/jit/compiler.h

@@ -8087,7 +8087,25 @@ private:
     // AVX: vector2f, 3f and 4f are all considered sub register SIMD types.
     bool isSubRegisterSIMDType(GenTreeSIMD* simdNode)
     {
-        return (simdNode->gtSIMDSize < getSIMDVectorRegisterByteLength());
+        unsigned vectorRegisterByteLength;
+#if defined(TARGET_XARCH)
+        // Calling the getSIMDVectorRegisterByteLength api causes the size of Vector<T> to be recorded
+        // with the AOT compiler, so that it cannot change from aot compilation time to runtime
+        // This api does not require such fixing as it merely pertains to the size of the simd type
+        // relative to the Vector<T> size as used at compile time. (So detecting a vector length of 16 here
+        // does not preclude the code from being used on a machine with a larger vector length.)
+        if (getSIMDSupportLevel() < SIMD_AVX2_Supported)
+        {
+            vectorRegisterByteLength = 16;
+        }
+        else
+        {
+            vectorRegisterByteLength = 32;
+        }
+#else
+        vectorRegisterByteLength = getSIMDVectorRegisterByteLength();
+#endif
+        return (simdNode->gtSIMDSize < vectorRegisterByteLength);
     }
 
     // Get the type for the hardware SIMD vector.