Remove uses of reg_mod from s390x (#5073)

Remove uses of reg_mod from the s390x backend. This required moving away from using r0/r1 as the result registers from a few different pseudo instructions, standardizing instead on r2/r3. That change was necessary as regalloc2 will not correctly allocate registers that aren't listed in the allocatable set, which r0/r1 are not.

Co-authored-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>
Co-authored-by: Chris Fallin <chris@cfallin.org>

cranelift/codegen/src/machinst/reg.rs

@@ -391,18 +391,6 @@ impl<'a, F: Fn(VReg) -> VReg> OperandCollector<'a, F> {
         }
     }
 
-    /// Add a register use+def, or "modify", where the reg must stay
-    /// in the same register on the input and output side of the
-    /// instruction.
-    pub fn reg_mod(&mut self, reg: Writable<Reg>) {
-        self.add_operand(Operand::new(
-            reg.to_reg().into(),
-            regalloc2::OperandConstraint::Reg,
-            regalloc2::OperandKind::Mod,
-            regalloc2::OperandPos::Early,
-        ));
-    }
-
     /// Add a register clobber set. This is a set of registers that
     /// are written by the instruction, so must be reserved (not used)
     /// for the whole instruction, but are not used afterward.