TLI: Remove DAG argument from getRegisterByName

TLI: Remove DAG argument from getRegisterByName

Replace with the MachineFunction. X86 is the only user, and only uses
it for the function. This removes one obstacle from using this in
GlobalISel. The other is the more tolerable EVT argument.

The X86 use of the function seems questionable to me. It checks hasFP,
before frame lowering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373292 91177308-0d34-0410-b5e6-96231b3b80d8