SIL: change way how a SILInstruction is marked as deleted

Instead of setting the parent pointer to null, set the `lastInitializedBitfieldID` to -1.
This allows to keep the parent block information, even when an instruction is removed from it's list.

include/swift/SIL/SILBasicBlock.h

@@ -85,7 +85,7 @@ private:
   ///    DD and EEE are uninitialized
   ///
   /// See also: SILBitfield::bitfieldID, SILFunction::currentBitfieldID.
-  uint64_t lastInitializedBitfieldID = 0;
+  int64_t lastInitializedBitfieldID = 0;
 
   // Used by `BasicBlockBitfield`.
   unsigned getCustomBits() const { return customBits; }