AArch64: Replace a RegScavenger instance with LivePhysRegs

findScratchNonCalleeSaveRegister() just needs a simple liveness
analysis, use LivePhysRegs for that as it is simpler and does not depend
on the kill flags.

This commit adds a convenience function available() to LivePhysRegs:
This function returns true if the given register is not reserved and
neither the register nor any of its aliases are alive.

Differential Revision: http://reviews.llvm.org/D21865

llvm-svn: 274685