GlobalISel: Add assert that legalize mutation makes sense
GlobalISel: Add assert that legalize mutation makes sense

I've repeatedly encountered bugs resulting from custom legalize
mutations returning nonsense legalize results, such as increasing the
number of elements for FewerElements. Add an assert function to make
sure the type to mutate to is consistent with the legalize action.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@352636 91177308-0d34-0410-b5e6-96231b3b80d8

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