[CallSite removal] Migrate all Alias Analysis APIs to use the newly
minted `CallBase` class instead of the `CallSite` wrapper.

This moves the largest interwoven collection of APIs that traffic in
`CallSite`s. While a handful of these could have been migrated with
a minorly more shallow migration by converting from a `CallSite` to
a `CallBase`, it hardly seemed worth it. Most of the APIs needed to
migrate together because of the complex interplay of AA APIs and the
fact that converting from a `CallBase` to a `CallSite` isn't free in its
current implementation.

Out of tree users of these APIs can fairly reliably migrate with some
combination of `.getInstruction()` on the `CallSite` instance and
casting the resulting pointer. The most generic form will look like `CS`
-> `cast_or_null<CallBase>(CS.getInstruction())` but in most cases there
is a more elegant migration. Hopefully, this migrates enough APIs for
users to fully move from `CallSite` to the base class. All of the
in-tree users were easily migrated in that fashion.

Thanks for the review from Saleem!

Differential Revision: https://reviews.llvm.org/D55641

llvm-svn: 350503
diff --git a/llvm/lib/Analysis/Loads.cpp b/llvm/lib/Analysis/Loads.cpp
index cd986dc..8129795 100644
--- a/llvm/lib/Analysis/Loads.cpp
+++ b/llvm/lib/Analysis/Loads.cpp
@@ -107,8 +107,8 @@
     return isDereferenceableAndAlignedPointer(ASC->getOperand(0), Align, Size,
                                               DL, CtxI, DT, Visited);
 
-  if (auto CS = ImmutableCallSite(V))
-    if (auto *RP = getArgumentAliasingToReturnedPointer(CS))
+  if (const auto *Call = dyn_cast<CallBase>(V))
+    if (auto *RP = getArgumentAliasingToReturnedPointer(Call))
       return isDereferenceableAndAlignedPointer(RP, Align, Size, DL, CtxI, DT,
                                                 Visited);