Rewrite the guts of the reassociate pass to be more efficient and logical. Instead

Rewrite the guts of the reassociate pass to be more efficient and logical.  Instead
of trying to do local reassociation tweaks at each level, only process an expression
tree once (at its root).  This does not improve the reassociation pass in any real way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@21768 91177308-0d34-0410-b5e6-96231b3b80d8