[IR] Don't accept nullptr as GEP element type

[nikic]

Summary

GetElementPtrInst::Create() (and IRBuilder methods based on it) currently accept nullptr as the element type, and will fetch the element type from the pointer in that case. Remove this fallback, as it is incompatible with opaque pointers. I've removed a handful of leftover calls using this behavior as a preliminary step.

Out-of-tree code affected by this change should either pass a proper type, or can temporarily explicitly call getPointerElementType(), if the newly added assertion is encountered.

[aeubanks]

awesome!

[stella.stamenova]

This change broke the windows and debian lldb bots:

https://lab.llvm.org/buildbot/#/builders/83/builds/8072
https://lab.llvm.org/buildbot/#/builders/68/builds/15070

llvm/include/llvm/IR/Instructions.h

protected:
   GetElementPtrInst *cloneImpl() const;
 
 public:
   static GetElementPtrInst *Create(Type *PointeeType, Value *Ptr,
                                    ArrayRef<Value *> IdxList,
                                    const Twine &NameStr = "",
                                    Instruction *InsertBefore = nullptr) {
     unsigned Values = 1 + unsigned(IdxList.size());
-    if (!PointeeType) {
-      PointeeType =
-          cast<PointerType>(Ptr->getType()->getScalarType())->getElementType();
-    } else {
-      assert(cast<PointerType>(Ptr->getType()->getScalarType())
-                 ->isOpaqueOrPointeeTypeMatches(PointeeType));
-    }
+    assert(PointeeType && "Must specify element type");
     return new (Values) GetElementPtrInst(PointeeType, Ptr, IdxList, Values,
                                           NameStr, InsertBefore);
   }
 
   static GetElementPtrInst *Create(Type *PointeeType, Value *Ptr,
                                    ArrayRef<Value *> IdxList,
                                    const Twine &NameStr,
                                    BasicBlock *InsertAtEnd) {
     unsigned Values = 1 + unsigned(IdxList.size());
-    if (!PointeeType) {
-      PointeeType =
-          cast<PointerType>(Ptr->getType()->getScalarType())->getElementType();
-    } else {
-      assert(cast<PointerType>(Ptr->getType()->getScalarType())
-                 ->isOpaqueOrPointeeTypeMatches(PointeeType));
-    }
+    assert(PointeeType && "Must specify element type");
     return new (Values) GetElementPtrInst(PointeeType, Ptr, IdxList, Values,
                                           NameStr, InsertAtEnd);
   }
 
   LLVM_ATTRIBUTE_DEPRECATED(static GetElementPtrInst *CreateInBounds(
         Value *Ptr, ArrayRef<Value *> IdxList, const Twine &NameStr = "",
         Instruction *InsertBefore = nullptr),
       "Use the version with explicit element type instead") {