(Merge to 6.8 branch after testing.)

There were a number of delicate interactions between RULEs and inlining
in GHC 6.6.  I've wanted to fix this for a long time, and some perf
problems in the 6.8 release candidate finally forced me over the edge!

The issues are documented extensively in OccurAnal, Note [Loop breaking
and RULES], and I won't duplicate them here.  (Many of the extra lines in
OccurAnal are comments!)

This patch resolves Trac bugs #1709, #1794, #1763, I believe.