Add 'override' to getRegisterByName in *ISelLowering.h

No functionality change intended.
Pass the value type to TLI::getRegisterByName

We must validate the value type in TLI::getRegisterByName, because if we
don't and the wrong type was used with the IR intrinsic, then we'll assert
(because we won't be able to find a valid register class with which to
construct the requested copy operation). For PPC64, additionally, the type
information is necessary to decide between the 64-bit register and the 32-bit
subregister.

No functionality change.
[PowerPC] Add global named register support

Support for the intrinsics that read from and write to global named registers
is added for r1, r2 and r13 (depending on the subtarget).