AssumptionCache: Update documentation comment.
AssumptionCache: Update documentation comment.

The comment was somewhat misleading in that it implied that passes were not
responsible for adding new assumptions to the assumption cache. This new
wording now explicitly mentions that they are required to do so.

Differential Revision: https://reviews.llvm.org/D29977

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@295148 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm