Move ILLink Perf to a separate job

Move ILLink Perf to a separate job

This changes ILLink performance testing to its own leg.
With this change, ILLink perf test will not be run for every PR by default.
It can be requested explicitly on a PR, and will run on the rolling builds.

JitBench build breaks have broken the PR builds a few times, so this change
moves ILLink perf tests out of the PR job, so that MusicStore testing can be
enabled again.

Commit migrated from https://github.com/dotnet/coreclr/commit/ef0fcc8b6f28d380cd193ca393557dc10aa0fb96