This refactors EffectControlLinearizer's LowerTruncateTaggedToBit and
LowerTruncateTaggedPointerToBit such that they share the common code.
This common code will grow further when supporting bigints in a future
CL.

R=jarin@chromium.org

Bug: 
Change-Id: I881d705de327243121b73e12fb93f2cd96f315f2
Reviewed-on: https://chromium-review.googlesource.com/771391
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49403}