[PATCH] [ValueTracking] Consider the bounds of PTX special registers

[Mark Heffernan]

I should also mention that I encountered some long compilation times which are superlinear with the unroll count when experimenting with the pragma loop limit.  With the current limit (32K) on a simple loop the compilation time is ~7s.  Doubling the limit results in a compilation time of ~50s.  It seems to be beneath llvm::UnrollLoop -> FoldBlockIntoPredecessor -> llvm::ScalarEvolution::forgetLoop.

http://reviews.llvm.org/D4150