Improve the LiveInterval class to keep track of which machine instruction

Improve the LiveInterval class to keep track of which machine instruction
defines each value# tracked by the interval.  This will be used to improve
coallescing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29830 91177308-0d34-0410-b5e6-96231b3b80d8