Fix a bug in which a node could be added to the

worklist twice: UpdateNodeOperands could morph
a new node into a node already on the worklist.
We would then recalculate the NodeId for this
existing node and add it to the worklist.  The
testcase is ARM/cse-libcalls.ll, the problem
showing up once UpdateNodeOperands is taught to
do CSE for calls.

llvm-svn: 58246