CodeGen: Use MachineInstr& more in MachineTraceMetrics, NFC
CodeGen: Use MachineInstr& more in MachineTraceMetrics, NFC

Push MachineInstr& through helper APIs for consistency.  This doesn't
remove any more implicit conversions, but it's a nice cleanup after
r274300.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@274301 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,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm