Fix register coalescer failure to prune value

Register coalescer fails for the test in the patch with the assertion in
JoinVals::ConflictResolution `DefMI != nullptr'. It attempts to join
live intervals for two adjacent instructions and erase the copy:

    %2:vreg_256 = COPY %1
    %3:vreg_256 = COPY killed %1

The LI needs to be adjusted to kill subrange for the erased instruction
and extend the subrange of the original def. That was done for the main
interval only but not for the subrange. As a result subrange had a VNI
pointing to the erased slot resulting in the above failure.

Differential Revision: https://reviews.llvm.org/D62162

llvm-svn: 361293