Also run the BranchElimination (plus CommonOperatorReducer and the
DeadCodeElimination) during the load elimination phase, so we can
elminate some Phi nodes early on that would otherwise confuse the
truncation analysis and/or representation selection, i.e. if there's a
branch that is never taken, that would yield undefined, we'd have to
choose tagged representation for the Phi, even if the always taken
branch yields an integer.

R=epertoso@chromium.org
BUG=v8:4930,v8:5141

Review-Url: https://codereview.chromium.org/2190543002
Cr-Commit-Position: refs/heads/master@{#38088}