[JumpThreading] Call eraseBlock when folding a conditional branch

This patch teaches the jump threading pass to call BPI->eraseBlock
when it folds a conditional branch.

Without this patch, BranchProbabilityInfo could end up with stale edge
probabilities for the basic block containing the conditional branch --
one edge probability with less than 1.0 and the other for a removed
edge.

Differential Revision: https://reviews.llvm.org/D92608