[OpenMPOpt] Allow indirect calls in AAKernelInfoCallSite (llvm#65836)

wq
The Attributor has gained support for indirect calls but it is opt-in.
This patch makes AAKernelInfoCallSite able to handle multiple potential
callees.

[LLVM][OpenMPOpt] Fix a crash when associated function is nullptr (llvm#66274)

The associated function can be a nullptr if it is an indirect call.
This causes a crash in `CheckCallee` which always assumes the callee
is a valid pointer.

Fix llvm#66904.

Change-Id: Ief6ea28456dc5c01709049153fe7e30f6aa4cb29

Author: jdoerfert