Don't accidentally leave small gaps in the live ranges when leaving the active

Don't accidentally leave small gaps in the live ranges when leaving the active
interval after an instruction. The leaveIntvAfter() method only adds liveness
from the instruction's boundary index to the inserted copy.

Ideally, SplitKit should be smarter about this, perhaps by combining useIntv()
and leaveIntvAfter() into one method that guarantees continuity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123858 91177308-0d34-0410-b5e6-96231b3b80d8