Fix optimizer bug affecting casts to char types.

When an expression that the intermediate code peephole optimizer could reduce to a constant was cast to a char type, the resulting value could be outside the range of that type.

The following program illustrates the problem:

#pragma optimize 1
#include <stdio.h>
int main(void) {
        int i = 0;
        i = (unsigned char)(i | -1);
        printf("%i\n", i);
}

DAG.pas

@@ -1166,6 +1166,12 @@ case op^.opcode of			{check for optimizations of this node}
                [cgByte,cgUByte,cgWord,cgUWord,cgLong,cgULong,cgReal,cgDouble,
                cgComp,cgExtended] then begin
                op^.left^.optype := totype.optype;
+               if totype.optype in [cgByte,cgUByte] then begin
+                  op^.left^.q := op^.left^.q & $00FF;
+                  if totype.optype = cgByte then
+                     if (op^.left^.q & $0080) <> 0 then
+                        op^.left^.q := op^.left^.q | $FF00;
+                  end; {if}
                opv := op^.left;
                end; {if}
          end {if}