LiveIntervals: add removeRegUnit

[nhaehnle]

Summary

See D22198 for the motivation: We have a pass that uses LiveIntervals anyway,
and there is now a requirement to track a physical register that is not
usually tracked at this point of the compilation. The pass also introduces
instructions that affect this physical register, but we want to preserve
LiveIntervals.

Rather than add brittle and rarely exercised code to keep the tracking of
the physical register intact, we want to just remove the corresponding
LiveRange -- it didn't exist before anyway, and subsequent passes don't
expect it to be there.

[MatzeB]

LGTM with nitpick:

include/llvm/CodeGen/LiveIntervalAnalysis.h

395–396

Please do not repeat the method name in the doxygen comment.

[MatzeB]

Please fix the doxygen comment (see also http://llvm.org/docs/CodingStandards.html#doxygen-use-in-documentation-comments).

include/llvm/CodeGen/LiveIntervalAnalysis.h

public:
     LiveRange *getCachedRegUnit(unsigned Unit) {
       return RegUnitRanges[Unit];
     }
 
     const LiveRange *getCachedRegUnit(unsigned Unit) const {
       return RegUnitRanges[Unit];
     }
 
+    /// removeRegUnit - Remove computed live range for Unit. Subsequent uses
+    /// should rely on on-demand recomputation.
+    void removeRegUnit(unsigned Unit) {
+      delete RegUnitRanges[Unit];
+      RegUnitRanges[Unit] = nullptr;
+    }
+
     /// Remove value numbers and related live segments starting at position
     /// @p Pos that are part of any liverange of physical register @p Reg or one
     /// of its subregisters.
     void removePhysRegDefAt(unsigned Reg, SlotIndex Pos);
 
     /// Remove value number and related live segments of @p LI and its subranges
     /// that start at position @p Pos.
     void removeVRegDefAt(LiveInterval &LI, SlotIndex Pos);