Move the expression test code into separate modules.

Move the expression test code into separate modules.
The Test() and TestInParens() functions do now return information about the
expression that was tested.
An if... statement where the expression is always true will now print a
warning "Unreachable code" if it has an else clause.

git-svn-id: svn://svn.cc65.org/cc65/trunk@2889 b7a2c559-68d2-44c3-8de9-860c34a00d81