[AArch64][DAGCombiner]: change -stop-after=isel to instruction-select
[AArch64][DAGCombiner]: change -stop-after=isel to instruction-select

"isel" is registered by AMDGPU. The test will break if the AMDGPU target
is not built.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@343553 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm