[CaptureTracking] Make MaxUsesToExplore cheaper (NFC)

The change in D78624 had a noticeable negative compile-time impact.
It seems that going through a function call for the MaxUsesToExplore
default is fairly expensive, at least if LLVM is not built with LTO.

This patch makes MaxUsesToExpore default to 0 and assigns the actual
default in the implementation instead. This recovers most of the
regression.

Differential Revision: https://reviews.llvm.org/D78734