UserspaceEmulator: Add a very simple instruction fetch cache

To avoid MMU region lookup on every single instruction fetch, we now
cache a raw pointer to the current instruction. This gets automatically
invalidated when we jump somewhere, but as long as we're executing
sequentially, instruction fetches will hit the cache and bypass all
the region lookup stuff.

This is about a ~2x speedup. :^)

DevTools/UserspaceEmulator/SoftCPU.h

@@ -56,7 +56,12 @@ public:
     void dump() const;
 
     u32 eip() const { return m_eip; }
-    void set_eip(u32 eip) { m_eip = eip; }
+    void set_eip(u32 eip)
+    {
+        m_eip = eip;
+        m_cached_code_ptr = nullptr;
+        m_cached_code_end = nullptr;
+    }
 
     struct Flags {
         enum Flag {
@@ -792,6 +797,8 @@ private:
     template<bool check_zf, typename Callback>
     void do_once_or_repeat(const X86::Instruction& insn, Callback);
 
+    void update_code_cache();
+
 private:
     Emulator& m_emulator;
 
@@ -800,6 +807,9 @@ private:
     u32 m_eflags { 0 };
 
     u32 m_eip { 0 };
+
+    const u8* m_cached_code_ptr { nullptr };
+    const u8* m_cached_code_end { nullptr };
 };
 
 }