fix crash in machine trace metrics due to processing dbg_value instructions (PR24199)

fix crash in machine trace metrics due to processing dbg_value instructions (PR24199)

The test in PR24199 ( https://llvm.org/bugs/show_bug.cgi?id=24199 ) crashes because machine
trace metrics was not ignoring dbg_value instructions when calculating data dependencies.

The machine-combiner pass asks machine trace metrics to calculate an instruction trace,
does some reassociations, and calls MachineInstr::eraseFromParentAndMarkDBGValuesForRemoval()
along with MachineTraceMetrics::invalidate(). The dbg_value instructions have their operands
invalidated, but the instructions are not expected to be deleted.

On a subsequent loop iteration of the machine-combiner pass, machine trace metrics would be
called again and die while accessing the invalid debug instructions.

Differential Revision: http://reviews.llvm.org/D11423

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@243057 91177308-0d34-0410-b5e6-96231b3b80d8