[InstSimplify] improve efficiency for detecting non-zero value

Stepping through callstacks in the example from D99759 reveals
this potential compile-time improvement.

The savings come from avoiding ValueTracking's computing known
bits if we have already dealt with special-case patterns.

Further improvements in this direction seem possible.

This makes a degenerate test based on PR49785 about 40x faster
(25 sec -> 0.6 sec), but it does not address the larger question
of how to limit computeKnownBitsFromAssume(). Ie, the original
test there is still infinite-time for all practical purposes.

Differential Revision: https://reviews.llvm.org/D100408