When we legalize target nodes, do not use getNode to create a new node,

use UpdateNodeOperands to just update the operands!  This is important because
getNode will allocate a new node if the node returns a flag and this breaks
assumptions in the legalizer that you can legalize some things multiple times
and get exactly the same results.

This latent bug was exposed by my ppc patch last night, and this fixes
gsm/toast.

llvm-svn: 28348