[SystemZ] Remove most hard-coded R1D instances for sibcalls

Indirect sibling calls need to use %r1 to hold the target address.
This is currently hard-coded in many places.  This is not only
unnecessary, but makes future changes in this area difficult.

This patch now encodes the target address as operand without
hard coding a register in most places throughout the MI back-end.
Code generation still always uses %r1, but this is now decided
solely in one place in SystemZTargetLowering::LowerCall.

NFC intended.