[JumpThreading] Safely replace uses of condition

This patch builds over https://reviews.llvm.org/rL303349 and replaces
the use of the condition only if it is safe to do so.

We should not blindly RAUW the condition if experimental.guard or assume
is a use of that
condition. This is because LVI may have used the guard/assume to
identify the
value of the condition, and RUAWing will fold the guard/assume and uses
before the guards/assumes.

Reviewers: sanjoy, reames, trentxintong, mkazantsev

Reviewed by: sanjoy, reames

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33257

llvm-svn: 303633