fhdl/tools: BUGFIX: fix group_by_target grouping

group_by_target does not properly combine target groups if statements
are presented in the order:

 ({A}, statement1)
 ({B}, statement2)
 ({A, B}, statement3)

which returns groups:

 ({A, B}, [statement1, statement3])
 ({B}, [statement2])

This patch fixes group_by_target such that the resulting group is:

 ({A, B}, [statement1, statement2, statement3])
fhdl/tools/group_by_target: remove resort_statements