Remove SequentialType from the type heirarchy.

Now that we have scalable vectors, there's a distinction that isn't
getting captured in the original SequentialType: some vectors don't have
a known element count, so counting the number of elements doesn't make
sense.

In some cases, there's a better way to express the commonality using
other methods. If we're dealing with GEPs, there's GEP methods; if we're
dealing with a ConstantDataSequential, we can query its element type
directly.

In the relatively few remaining cases, I just decided to write out
the type checks. We're talking about relatively few places, and I think
the abstraction doesn't really carry its weight. (See thread "[RFC]
Refactor class hierarchy of VectorType in the IR" on llvmdev.)

Differential Revision: https://reviews.llvm.org/D75661

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -2505,7 +2505,11 @@ Error BitcodeReader::parseConstants() {
       if (Record.empty())
         return error("Invalid record");
 
-      Type *EltTy = cast<SequentialType>(CurTy)->getElementType();
+      Type *EltTy;
+      if (auto *Array = dyn_cast<ArrayType>(CurTy))
+        EltTy = Array->getElementType();
+      else
+        EltTy = cast<VectorType>(CurTy)->getElementType();
       if (EltTy->isIntegerTy(8)) {
         SmallVector<uint8_t, 16> Elts(Record.begin(), Record.end());
         if (isa<VectorType>(CurTy))