Revert "Add support for fetching signed values from tagged pointers."

This reverts commit 4d9039c8.

This is causing the greendragon bots to fail most of the time when
running TestNSDictionarySynthetic.py.  Reverting until Jim has a chance
to look at this on Monday.  Running the commands from that test from
the command line, it fails 10-13% of the time on my desktop.

This is a revert of Jim's changes in https://reviews.llvm.org/D99694