misched: Fix LiveInterval update to better handle DebugVal.

Assertion failed: (itr != mi2iMap.end() && "Instruction not found in maps.")
rdar://12777252.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169070 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed