ir/value: Add an IsZero() member function to Value's interface

By far, one of the most common things to check for is whether or not a
value is zero, as it typically allows folding away unnecesary
operations (other close contenders that can help with eliding operations  are 1 and -1).

So instead of requiring a check for an immediate and then actually
retrieving the integral value and checking it, we can wrap it within a
function to make it more convenient.

src/frontend/ir/value.cpp

@@ -194,4 +194,8 @@ bool Value::HasAllBitsSet() const {
     }
 }
 
+bool Value::IsZero() const {
+    return IsImmediate() && GetImmediateAsU64() == 0;
+}
+
 } // namespace Dynarmic::IR

src/frontend/ir/value.h

@@ -82,6 +82,15 @@ public:
      */
     bool HasAllBitsSet() const;
 
+    /**
+     * Whether or not the current value contains a representation of zero.
+     *
+     * Note that this function will always return false if the contained
+     * value is not a a constant value. In other words, if IsImmediate()
+     * would return false on an instance, then so will this function.
+     */
+    bool IsZero() const;
+
 private:
     Type type;