MachineRegisterInfo: Introduce isPhysRegUsed()

This method checks whether a physical regiser or any of its aliases are
used in the function.

Using this function in SIRegisterInfo::findUnusedReg() should also fix
this reported failure:

http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20150803/292143.html
http://reviews.llvm.org/rL242173#inline-533

The report doesn't come with a testcase and I don't know enough about
AMDGPU to create one myself.

llvm-svn: 245329