Avoid doing explicit cast in simulator-a64::get_sp().

src/a64/simulator-a64.h

Index: src/a64/simulator-a64.h
diff --git a/src/a64/simulator-a64.h b/src/a64/simulator-a64.h
index 6279677ca8565e46f32d4ce8caafed06a3d6b1b7..b898e3ff15c0c11f3328fded4f5bd800a91b61df 100644
--- a/src/a64/simulator-a64.h
+++ b/src/a64/simulator-a64.h
@@ -445,7 +445,7 @@ class Simulator : public DecoderVisitor {
   }
   Instruction* lr() { return reg<Instruction*>(kLinkRegCode); }
 
-  Address get_sp() { return reinterpret_cast<Address>(sp()); }
+  Address get_sp() { return reg<Address>(31, Reg31IsStackPointer); }
 
   // Return 'size' bits of the value of a floating-point register, as the
   // specified type. The value is zero-extended to fill the result.