[opaque pointer type] Avoid using PointerType::getElementType when parsing IR

A few calls are left in for error checking - but I'm commenting those
out & trying to build some IR tests (aiming for Argument Promotion to
start with). When I get any of these tests passing I may add flag to
disable the checking so I can add tests that pass with the assertion in
place.

llvm-svn: 234206
diff --git a/llvm/lib/IR/Function.cpp b/llvm/lib/IR/Function.cpp
index 33e1526..8275cc6 100644
--- a/llvm/lib/IR/Function.cpp
+++ b/llvm/lib/IR/Function.cpp
@@ -215,9 +215,7 @@
   return getType()->getContext();
 }
 
-FunctionType *Function::getFunctionType() const {
-  return cast<FunctionType>(getType()->getElementType());
-}
+FunctionType *Function::getFunctionType() const { return Ty; }
 
 bool Function::isVarArg() const {
   return getFunctionType()->isVarArg();
@@ -242,7 +240,8 @@
 Function::Function(FunctionType *Ty, LinkageTypes Linkage, const Twine &name,
                    Module *ParentModule)
     : GlobalObject(PointerType::getUnqual(Ty), Value::FunctionVal, nullptr, 0,
-                   Linkage, name) {
+                   Linkage, name),
+      Ty(Ty) {
   assert(FunctionType::isValidReturnType(getReturnType()) &&
          "invalid return type");
   setIsMaterializable(false);