Eliminate the loop that searches through each of the operands

of each use in the SelectionDAG ReplaceAllUses* functions. Thanks
to Chris for spotting this opportunity.

Also, factor out code from all 5 of the ReplaceAllUses* functions
into AddNonLeafNodeToCSEMaps, which is now renamed
AddModifiedNodeToCSEMaps to more accurately reflect its purpose.

llvm-svn: 62964