Fix crash or error in certain cases when using common subexpression elimination.

In certain rare cases, constant subexpression elimination could set the left subtree of a pc_bno operation in the intermediate code to nil. This could lead to null pointer dereferences, sometimes resulting in a crash or error during native code generation.

The below program sometimes demonstrates the problem (dependent on zero page contents):

#pragma optimize 16
struct F {int *p;};
void foo(struct F* f)
{
    struct {int c;} s = {0};
    ++f->p;
    s.c |= *--f->p;
}

DAG.pas

@@ -2531,6 +2531,11 @@ var
             end {if}
          else
             op2 := op3^.left;
+         if op2 = nil then begin
+            op2 := pointer(Calloc(sizeof(intermediate_code)));
+            op2^.opcode := pc_nop;
+            op2^.optype := cgWord;
+            end; {if}
          op1^.left := op3;			{place in the new location}
          end; {Combine}