major surgery on tblgen: generalize TreePatternNode

major surgery on tblgen: generalize TreePatternNode
to maintain a list of types (one for each result of
the node) instead of a single type.  There are liberal
hacks added to emulate the old behavior in various
situations, but they can start disolving now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98999 91177308-0d34-0410-b5e6-96231b3b80d8