Fixed a problem in the optimizer function that rewrites ops that use the

Fixed a problem in the optimizer function that rewrites ops that use the
stack. Useless instructions (duplicate loads or transfers) within the sequence
are left intact and may cause problems because the interfere with the
replacement code. A run of OptRemoveUnusedLoads and friends should fix the
problem, bit this step may be disabled, so the routine has to check for this
condition and avoid it (by not doing the replacement).

git-svn-id: svn://svn.cc65.org/cc65/trunk@4551 b7a2c559-68d2-44c3-8de9-860c34a00d81