RegisterCoalescer: Improve previous fix for wrong def after.

The previous fix in r225503 was needlessly complicated. The problem goes
away as well if the arguments to MergeValueNumberInto are supplied in the
correct order.
This was previously missed because the existing code already had the
wrong order but an additional later Merge was hiding the bug for the
main liverange VNI.

llvm-svn: 229424