The old-fashioned many-to-one value mapping doesn't always work when
merging vector lanes. A value can map to multiple different values, and
it can even be necessary to insert new PHIs.

When a value number is defined by a copy from a value number that
required SSa update, include the live range of the copied value number
in the SSA update as well. It is not necessarily a copy of the original
value number any longer.

llvm-svn: 164329