emit_x64_vector: Avoid left shift of negative value in LogicalVShift

Now that we handle the signed variants, we also have to be careful about left shifts with negative values,
as this is considered undefined behavior.

src/backend_x64/emit_x64_vector.cpp

@@ -945,7 +945,8 @@ static constexpr T LogicalVShift(T x, T y) {
         return x >> T(-shift_amount);
     }
 
-    return x << T(shift_amount);
+    using unsigned_type = std::make_unsigned_t<T>;
+    return static_cast<T>(static_cast<unsigned_type>(x) << static_cast<unsigned_type>(shift_amount));
 }
 
 void EmitX64::EmitVectorLogicalVShiftS8(EmitContext& ctx, IR::Inst* inst) {