[LoopPred] Stop passing around builders [NFC]

This is a preparatory patch for D60093. This patch itself is NFC, but while preparing this I noticed and committed a small hoisting change in rL358419.

The basic structure of the new scheme is that we pass around the guard ("the using instruction"), and select an optimal insert point by examining operands at each construction point. This seems conceptually a bit cleaner to start with as it isolates the knowledge about insertion safety at the actual insertion point.

Note that the non-hoisting path is not actually used at the moment. That's not exercised until D60093 is rebased on this one.

Differential Revision: https://reviews.llvm.org/D60718

llvm-svn: 358434