Replace some uses of getSubtargetImpl with the cached version

Replace some uses of getSubtargetImpl with the cached version
off of the MachineFunction or with the version that takes a
Function reference as an argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227185 91177308-0d34-0410-b5e6-96231b3b80d8