[InstCombine] Replace an unnecessary use of a matcher with just an isa and a cast. NFC
[InstCombine] Replace an unnecessary use of a matcher with just an isa and a cast. NFC

We aren't looking through any levels of IR here so I don't think we need the power of a matcher or the temporary variable it requires.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@306885 91177308-0d34-0410-b5e6-96231b3b80d8

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