[CallSite removal] Migrate the statepoint GC infrastructure to use the
`CallBase` class rather than `CallSite` wrappers.

I pushed this change down through most of the statepoint infrastructure,
completely removing the use of CallSite where I could reasonably do so.
I ended up making a couple of cut-points: generic call handling
(instcombine, TLI, SDAG). As soon as it hit truly generic handling with
users outside the immediate code, I simply transitioned into or out of
a `CallSite` to make this a reasonable sized chunk.

Differential Revision: https://reviews.llvm.org/D56122

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353660 91177308-0d34-0410-b5e6-96231b3b80d8
11 files changed