[ValueTracking] Make unit tests easier to write; NFC

Generalize the existing MatchSelectPatternTest class to also work
with other types of tests. This reduces the amount of boilerplate
necessary to write ValueTracking tests in general, and computeKnownBits
tests in particular.

The inherited convention is that the function must be @test and the
tested instruction %A.

Differential Revision: https://reviews.llvm.org/D55141

llvm-svn: 348043