Have the selection dag grab TargetLowering off of the subtarget
inside init rather than have it passed in as an argument.
Reset the target options and optimization level as the first
thing we do inside selection dag. This code needs to be
migrated to queries on the function rather than global
data, but this organizes things before we start grabbing
the subtarget.
Disallow using function parameters in extended asm inputs or outputs in naked functions (PR21178)

Clang won't emit any prologues for such functions, so it would assert trying to
codegen the parameter references.

This patch makes Clang check the extended asm inputs and outputs for
references to function parameters.

Differential Revision: http://reviews.llvm.org/D5640
Cache SelectionDAGISel TargetInstrInfo lookups on the class and
propagate. Also use the TargetSubtargetInfo and the MachineFunction
and move TargetRegisterInfo query closer to uses.