Let += and -= operators in BranchProbability have saturation behaviors.
Let += and -= operators in BranchProbability have saturation behaviors.

This commit is for a later patch that is depend on it. The sum of two
branch probabilities can be greater than 1 due to rounding. It is safer
to saturate the results of sum and subtraction.




git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@253421 91177308-0d34-0410-b5e6-96231b3b80d8

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