[EarlyCSE] Simplify guard intrinsics

Summary:
This change teaches EarlyCSE some basic properties of guard intrinsics:

 - Guard intrinsics read all memory, but don't write to any memory
 - After a guard has executed, the condition it was guarding on can be
   assumed to be true
 - Guard intrinsics on a constant `true` are no-ops

Reviewers: reames, hfinkel

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19578

llvm-svn: 268120