Don't optimize out certain volatile stores.

This could occur due to the new native-code peephole optimizations for stz instructions, which can collapse consecutive identical ones down to one instruction. This is OK most of the time, but not when dealing with volatile variables, so disable it in that case.

The following test case shows the issue (look at the generated code):

#pragma optimize -1
volatile int a;
int main(void) {
        a = 0;
        a = 0;
}

Native.pas

@@ -1744,12 +1744,14 @@ var
             if npeep[ns].opcode = npeep[ns+1].opcode then
                if npeep[ns].operand = npeep[ns+1].operand then
                   if npeep[ns].name = npeep[ns+1].name then
-                     Remove(ns+1);
+                     if not volatile then
+                        Remove(ns+1);
          
          m_stz_dir, m_stz_dirX:
             if npeep[ns].opcode = npeep[ns+1].opcode then
                if npeep[ns].operand = npeep[ns+1].operand then
-                  Remove(ns+1);
+                  if not volatile then
+                     Remove(ns+1);
 
          otherwise: ;