Identified with bugprone-unused-local-non-trivial-variable.

There's a pattern throughout LLVM of cl::opts being exported. That in
itself is probably a bit unfortunate, but what's especially bad about it
is that a lot of those symbols are in the global namespace. Move them
into the llvm namespace.

While doing this, I noticed some other variables in the global namespace
and moved them as well.

The main difference between SimplifyCFG's `setBranchWeights`​ and the ProfDataUtils' is that the former doesn't propagate all-zero weights. That seems like a sensible thing to do, so updated the latter accordingly, and added a flag to control the behavior.

Also moved to ProfDataUtils the logic fitting 64-bit weights to 32-bit.

As side-effect, this fixes some profcheck failures.

The split in this code path was left over from when we had to support
the old PM and the new PM at the same time. Now that the legacy pass has
been dropped, this simplifies the code a little bit and swaps pointers
for references in a couple places.

Reviewers: aeubanks, efriedma-quic, wlei-llvm

Reviewed By: aeubanks

Pull Request: https://github.com/llvm/llvm-project/pull/159858

See https://discourse.llvm.org/t/rfc-keep-globalvalue-guids-stable/84801
for context.

This is a non-functional change which just changes the interface of
GlobalValue, in preparation for future functional changes. This part
touches a fair few users, so is split out for ease of review. Future
changes to the GlobalValue implementation can then be focused purely on
that class.

This does the following:

* Rename GlobalValue::getGUID(StringRef) to
  getGUIDAssumingExternalLinkage. This is simply making explicit at the
  callsite what is currently implicit.
* Where possible, migrate users to directly calling getGUID on a
  GlobalValue instance.
* Otherwise, where possible, have them call the newly renamed
  getGUIDAssumingExternalLinkage, to make the assumption explicit.


There are a few cases where neither of the above are possible, as the
caller saves and reconstructs the necessary information to compute the
GUID themselves. We want to migrate these callers eventually, but for
this first step we leave them be.

Clean the unneeded field 'TotalCollectedSamples' and the unnecessary
loop.
The field seems introduced in:https://reviews.llvm.org/D31952, and its
uses were removed in: https://reviews.llvm.org/D19287, but this field
and unnecessary calculation were not cleaned up.
This patch will remove these unneeded codes.