An idea to speed up cse

[Joern Rennecke <amylaar at cygnus dot co dot uk>]

I think a weak point of cse is that remove_invalid_refs has to go
through every entry in the hash table to find out which expressions
refer to a particular register.
I propose that for every register in the table, there should be a doubly
linked list of non-register table entries that refer to that register.
then remove_invalid_refs has to go only through that list.
The elements of this list would also have two more pointers so that
every non-register table entry has a doubly-linked list of list entries
that refer to it.