[IR] Add a few asserts to provide a better failure signature if you try to create a load/store/alloca with no alignment or insertion position

[craig.topper]

Summary

If no alignment is specified we try to find the datalayout by using the insert position to get the module so we can get the datalayout. But if those are null, then we deference a null pointer.

This patch adds asserts to make the failure a little more obvious than just seg faulting.

[efriedma]

LGTM with one minor comment

llvm/lib/IR/Instructions.cpp

1265

Maybe also assert BB->getParent() is non-null? Someone mentioned an issue in some downstream code where the function was null.

llvm/lib/IR/Instructions.cpp

assert(!isa<BasicBlock>(Amt) &&
            "Passed basic block into allocation size parameter! Use other ctor");
     assert(Amt->getType()->isIntegerTy() &&
            "Allocation array size is not an integer!");
   }
   return Amt;
 }
 
 static Align computeAllocaDefaultAlign(Type *Ty, BasicBlock *BB) {
+  assert(BB && "Insertion BB cannot be null when alignment not provided!");
   const DataLayout &DL = BB->getModule()->getDataLayout();
   return DL.getPrefTypeAlign(Ty);
 }
 
 static Align computeAllocaDefaultAlign(Type *Ty, Instruction *I) {
+  assert(I && "Insertion position cannot be null when alignment not provided!");
   return computeAllocaDefaultAlign(Ty, I->getParent());
 }
 
 AllocaInst::AllocaInst(Type *Ty, unsigned AddrSpace, const Twine &Name,
                        Instruction *InsertBefore)
   : AllocaInst(Ty, AddrSpace, /*ArraySize=*/nullptr, Name, InsertBefore) {}
 
 AllocaInst::AllocaInst(Type *Ty, unsigned AddrSpace, const Twine &Name,

 void LoadInst::AssertOK() {
   assert(getOperand(0)->getType()->isPointerTy() &&
          "Ptr must have pointer type.");
   assert(!(isAtomic() && getAlignment() == 0) &&
          "Alignment required for atomic load");
 }
 
 static Align computeLoadStoreDefaultAlign(Type *Ty, BasicBlock *BB) {
+  assert(BB && "Insertion BB cannot be null when alignment not provided!");
   const DataLayout &DL = BB->getModule()->getDataLayout();
   return DL.getABITypeAlign(Ty);
 }
 
 static Align computeLoadStoreDefaultAlign(Type *Ty, Instruction *I) {
+  assert(I && "Insertion position cannot be null when alignment not provided!");
   return computeLoadStoreDefaultAlign(Ty, I->getParent());
 }
 
 LoadInst::LoadInst(Type *Ty, Value *Ptr, const Twine &Name,
                    Instruction *InsertBef)
     : LoadInst(Ty, Ptr, Name, /*isVolatile=*/false, InsertBef) {}
 
 LoadInst::LoadInst(Type *Ty, Value *Ptr, const Twine &Name,