Change the ValueList array for each node to be shared instead of individually

Change the ValueList array for each node to be shared instead of individually
allocated.  Further, in the common case where a node has a single value, just
reference an element from a small array.  This is a small compile-time wi.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24250 91177308-0d34-0410-b5e6-96231b3b80d8