Don't remove symbols or otherwise simplify expressions while assembly is

Don't remove symbols or otherwise simplify expressions while assembly is
still in progress. There may be information that is needed, and when
assembly is done it is still time to do so. (Needs more work).
Better expression checks for fragments. Stuff that was detected by the
linker before is now handled by the assembler.

git-svn-id: svn://svn.cc65.org/cc65/trunk@2700 b7a2c559-68d2-44c3-8de9-860c34a00d81