[CallSite removal] Move the verifier to use `CallBase` instead of the `CallSite` wrapper.
[CallSite removal] Move the verifier to use `CallBase` instead of the
`CallSite` wrapper.

Mostly mechanical, but I've tried to tidy up code where it made sense to
do so.

Differential Revision: https://reviews.llvm.org/D56143

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@350507 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm