Add x86-specific instruction for i64x2 multiplication

Without this special instruction, legalizing to the AVX512 instruction AND the SSE instruction sequence is impossible. This extra instruction would be rendered unnecessary by the x64 backend.

cranelift/codegen/meta/src/isa/x86/instructions.rs

@@ -532,6 +532,23 @@ pub(crate) fn define(
         .operands_out(vec![a]),
     );
 
+    let x = &Operand::new("x", I64x2);
+    let y = &Operand::new("y", I64x2);
+    let a = &Operand::new("a", I64x2);
+    ig.push(
+        Inst::new(
+            "x86_pmullq",
+            r#"
+        Multiply Packed Integers -- Multiply two 64x2 integers and receive a 64x2 result with
+        lane-wise wrapping if the result overflows. This instruction is necessary to add distinct
+        encodings for CPUs with newer vector features.
+        "#,
+            &formats.binary,
+        )
+        .operands_in(vec![x, y])
+        .operands_out(vec![a]),
+    );
+
     let x = &Operand::new("x", TxN);
     let y = &Operand::new("y", TxN);
     let f = &Operand::new("f", iflags);