[EarlyCSE] Remove guards with conditions known to be true

If a condition is calculated only once, and there are multiple guards on this condition, we should be able
to remove all guards dominated by the first of them. This patch allows EarlyCSE to try to find the condition
of a guard among the known values, and if it is true, remove the guard. Otherwise we keep the guard and
mark its condition as 'true' for future consideration.

Reviewers: sanjoy, reames, apilipenko, skatkov, anna, dberlin

Reviewed By: reames, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32476

llvm-svn: 301623