MkFitCore: move simd pragmas to inner loops to address clang warnings

[dan131riley]

PR description:

Checking the vectorization diagnostics revealed that gcc was only vectorizing the innermost two nested loops, out of four. Apparently LLVM is pickier about the locality of pragma simd, as moving the directive to the loops that are actually optimized by gcc allowed LLVM to vectorize successfully.

PR also adds constexpr to a few if statements on config constants.

Resolves #43886

PR validation:

Compiles with no warnings with clang and gcc. Purely technical, no physics changes expected.

[cmsbuild]

cms-bot internal usage

[cmsbuild]

+code-checks

Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-43915/38765

• This PR adds an extra 52KB to repository

[cmsbuild]

A new Pull Request was created by @dan131riley (Dan Riley) for master.

It involves the following packages:

• RecoTracker/MkFitCore (reconstruction)

@jfernan2, @cmsbuild, @mandrenguyen can you please review it and eventually sign? Thanks.
@rovere, @felicepantaleo, @dgulhan, @VourMa, @gpetruc, @VinInn, @missirol, @GiacomoSguazzoni, @mtosi, @JanFSchulte, @makortel, @mmusich this is something you requested to watch as well.
@sextonkennedy, @rappoccio, @antoniovilela you are the release manager for this.

cms-bot commands are listed here

[jfernan2]

please test

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-3ccb2d/37326/summary.html
COMMIT: 02ea183
CMSSW: CMSSW_14_1_X_2024-02-09-1100/el8_amd64_gcc12
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week1/cms-sw/cmssw/43915/37326/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

Summary:

• You potentially removed 26 lines from the logs
• Reco comparison results: 47 differences found in the comparisons
• DQMHistoTests: Total files compared: 48
• DQMHistoTests: Total histograms compared: 3248626
• DQMHistoTests: Total failures: 3
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3248601
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 47 files compared)
• Checked 200 log files, 161 edm output root files, 48 DQM output files
• TriggerResults: no differences found

[jfernan2]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next master IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @sextonkennedy, @rappoccio, @antoniovilela (and backports should be raised in the release meeting by the corresponding L2)

[antoniovilela]

+1