Fix a FIXME: in ReplaceNodeWith, if the new node

Fix a FIXME: in ReplaceNodeWith, if the new node
is morphed by AnalyzeNewNode into a previously
processed node, and different result values of
that node are remapped to values with different
nodes, then we could end up using wrong values
here [we were assuming that all results remap
to values with the same underlying node].  This
seems theoretically possible, but I don't have
a testcase.  The meat of the patch is in the
changes to AnalyzeNewNode/AnalyzeNewValue and
ReplaceNodeWith.  While there, I changed names
like RemapNode to RemapValue, since it really
remaps values.  To tell the truth, I would be
much happier if we were only remapping nodes
(it would simplify a bunch of logic, and allow
for some cute speedups) but I haven't yet worked
out how to do that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@58372 91177308-0d34-0410-b5e6-96231b3b80d8