CodeGen: Handle missed case of block removal during BlockPlacement.
CodeGen: Handle missed case of block removal during BlockPlacement.

There is a use after free bug in the existing code. Loop layout selects
a preferred exit block, and then lays out the loop. If this block is
removed during layout, it needs to be invalidated to prevent a use after
free.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@285348 91177308-0d34-0410-b5e6-96231b3b80d8

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