[AArch64][GlobalISel] Make LR livein to entry in llvm.returnaddress selection

This fixes a couple verifier failures on this bot:

http://green.lab.llvm.org/green/job/test-suite-verify-machineinstrs-aarch64-globalisel-O0-g/

The failures show up in eeprof-1.c and pr17377.c in the GCC C Torture Suite.

Specifically:

*** Bad machine code: MBB has allocatable live-in, but isn't entry or landing-pad. ***
- function:    foo
- basic block: %bb.3 if.end (0x7fac7106dfc8)
- p. register: $lr

and

*** Bad machine code: Using an undefined physical register ***
- function:    f
- basic block: %bb.1 entry (0x7f8941092588)
- instruction: %18:gpr64 = COPY $lr
- operand 1:   $lr

Unlike SDAG, we were setting LR as a live in to the block containing the
returnaddress.

Also, this ensures that we don't add LR as a livein to the entry block twice.
In MachineBasicBlock.h there's a comment saying

"Note that it is an error to add the same register to the same set more than
once unless the intention is to call sortUniqueLiveIns after all registers are
added."

so it's probably good to avoid adding LR twice.

Surprisingly the verifier doesn't complain about that. Maybe it should.

Differential Revision: https://reviews.llvm.org/D79657