LiveInterval: Fix computeFromMainRange() producing adjacent segments with same valno

If two livesegments from different subranges happened to have the same
definition they could possibly end up as two adjacent segments in the
main liverange with the same value number which is not allowed. Detect
such cases and fix them in the 2nd pass of computeFromMainRange() if
necessary.

No testcase as there is only an out-of-tree target where I can sensibly
come up with one.

llvm-svn: 234382