[SamplePGO] Add a cutoff for number of profile matching anchors (#95542)

The algorithm added by PR #87375 can be potentially quadratic in the
number of anchors. This is almost never a problem because normally
functions have a reasonable number of function calls.

However, in some rare cases of auto-generated code we observed very
large functions that trigger quadratic behaviour here (resulting in
>130GB of peak heap memory usage for clang). Let's add a knob for
controlling the max number of callsites in a function above which stale
profile matching won't be performed.

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