[IR] Redesign the case iterator in SwitchInst to actually be an iterator
and to expose a handle to represent the actual case rather than having
the iterator return a reference to itself.

All of this allows the iterator to be used with common STL facilities,
standard algorithms, etc.

Doing this exposed some missing facilities in the iterator facade that
I've fixed and required some work to the actual iterator to fully
support the necessary API.

Differential Revision: https://reviews.llvm.org/D31548

llvm-svn: 300032
diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index d61684b..faa5ed0 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -3659,16 +3659,16 @@
   // Initialize some new operands.
   assert(OpNo+1 < ReservedSpace && "Growing didn't work!");
   setNumHungOffUseOperands(OpNo+2);
-  CaseIt Case(this, NewCaseIdx);
+  CaseHandle Case(this, NewCaseIdx);
   Case.setValue(OnVal);
   Case.setSuccessor(Dest);
 }
 
 /// removeCase - This method removes the specified case and its successor
 /// from the switch instruction.
-SwitchInst::CaseIt SwitchInst::removeCase(CaseIt i) {
-  unsigned idx = i.getCaseIndex();
-  
+SwitchInst::CaseIt SwitchInst::removeCase(CaseIt I) {
+  unsigned idx = I->getCaseIndex();
+
   assert(2 + idx*2 < getNumOperands() && "Case index out of range!!!");
 
   unsigned NumOps = getNumOperands();