TargetRegisterInfo: Provide a way to check assigned registers in getRegAllocationHints()

TargetRegisterInfo: Provide a way to check assigned registers in getRegAllocationHints()

Pass a const reference to LiveRegMatrix to getRegAllocationHints()
because some targets can prodive better hints if they can test whether a
physreg has been used for register allocation yet.

llvm-svn: 242340