[BPF] Permit all user instructed offset relocatiions

Currently, not all user specified relocations
(with clang intrinsic __builtin_preserve_access_index())
will turn into relocations.

In the current implementation, a __builtin_preserve_access_index()
chain is turned into relocation only if the result of the clang
intrinsic is used in a function call or a nonzero offset computation
of getelementptr. For all other cases, the relocatiion request
is ignored and the __builtin_preserve_access_index() is turned
into regular getelementptr instructions.
The main reason is to mimic bpf_probe_read() requirement.

But there are other use cases where relocatable offset is
generated but not used for bpf_probe_read(). This patch
relaxed previous constraints when to generate relocations.
Now, all user __builtin_preserve_access_index() will have
relocations generated.

Differential Revision: https://reviews.llvm.org/D67688

llvm-svn: 372198