[PredicateInfo] Factor out PredicateInfoBuilder (NFC)

When running IPSCCP on a module with many small functions, memory
usage is dominated by PredicateInfo, which is a huge structure
(partially due to some unfortunate nested SmallVector use). However,
most of it is actually only temporary state needed to build
predicate info, and does not need to be retained after initial
construction.

This patch factors out the predicate building logic and state
into a separate PrediceInfoBuilder, with the extra bonus that
it does not need to live in the header anymore.

Differential Revision: https://reviews.llvm.org/D78326