Add support to tablegen for naming the nodes themselves, not just the operands,

Add support to tablegen for naming the nodes themselves, not just the operands,
in selectiondag patterns.  This is required for the upcoming shuffle_vector rewrite,
and as it turns out, cleans up a hack in the Alpha instruction info.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@67286 91177308-0d34-0410-b5e6-96231b3b80d8