Rewrite local-single-store-elim to not create large data structures.

The local-single-store-elim algorithm is not fundamentally bad.
However, when there are a large number of variables, some of the
maps that are used can become very large.  These large data structures
then take a very long time to be destroyed.  I've seen cases around 40%
if the time.

I've rewritten that algorithm to not use as much memory.  This give a
significant improvement when running a large number of shader through
DXC.

I've also made a small change to local-single-block-elim to delete the
loads that is has replaced.  That way local-single-store-elim will not
have to look at those.  local-single-store-elim now does the same thing.

The time for one set goes from 309s down to 126s.  For another set, the
time goes from 102s down to 88s.