[Backend] DBG_CALLSITE & DBG_CALLSITEPARAM instr handling

[djtodoro]

Summary

Handle DBG_CALLSITE and DBG_CALLSITEPARAM meta instructions through backend pipeline. It partially relies on the way DBG_VALUE is handled.

Authors: @asowda, @NikolaPrica, @djtodoro, @ivanbaev

[djtodoro]

• Rename: VariableNotChanged ===> ArgumentNotModified
• Refactor test cases

[dstenb]

lib/CodeGen/StackColoring.cpp

963

The iterator does seem to be invalidated here (or am I overlooking something?), so you should be able to use BB.instrs() here instead.

lib/CodeGen/StackSlotColoring.cpp

376

MBB.instrs()

[dstenb]

lib/CodeGen/StackColoring.cpp

963

Gah, "does _not_".

[djtodoro]

@dstenb Thanks for comments!

lib/CodeGen/StackColoring.cpp

963

Yes.

lib/CodeGen/StackSlotColoring.cpp

376

As well as above.

[djtodoro]

• Use MBB.instrs()