glsl: change opt_copy_propagation_elements data structures

glsl: change opt_copy_propagation_elements data structures

Instead of keeping multiple acp_entries in lists, have a single
acp_entry per variable. With this, the implementation of clone is more
convenient and now fully implemented. In the previous code, clone was
only partial.

Before this patch, each acp_entry struct represented a write to a
variable including LHS, RHS and a mask of what channels were written
to. There were two main hash tables, the first (lhs_ht) stored a list
of acp_entries per LHS variable, with the values available to copy for
that variable; the second (rhs_ht) was a "reverse index" for the first
hash table, so stored acp_entries per RHS variable.

After the patch, there's a single acp_entry struct per LHS variable,
it contains an array with references to the RHS variables per
channel. There now is a single hash table, from LHS variable to the
corresponding entry. The "reverse index" is stored in the ACP entry,
in the form of a set of variables that copy from the LHS. To make the
clone operation cheaper, the ACP entries are created on demand.

This should not change the result of copy propagation, a later patch
will take advantage of the clone operation.

v2: Add note clarifying how the hashtable is destroyed.

v3: (all from Eric Anholt)
    Add remove_unused_var_from_dsts() function for reuse.
    Remove from dsts as we go instead of clearing at the end.
    Add clarifying comment to erase().

Reviewed-by: Eric Anholt <eric@anholt.net>