Revert "cmd/compile: improve register overwrite decision for resultInArg0 ops"

This reverts CL 460656
Reason for revert: This breaks s390x.

Change-Id: I8fada14fabc90593b8033ed11188c04963d2da75
Reviewed-on: https://go-review.googlesource.com/c/go/+/463475
Reviewed-by: Matthew Dempsky <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Keith Randall <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/regalloc.go

@@ -1544,27 +1544,13 @@ func (s *regAllocState) regalloc(f *Func) {
 						}
 					}
 				}
-
 				// Avoid future fixed uses if we can.
 				if m&^desired.avoid != 0 {
 					m &^= desired.avoid
 				}
 				// Save input 0 to a new register so we can clobber it.
 				c := s.allocValToReg(v.Args[0], m, true, v.Pos)
 				s.copies[c] = false
-
-				// Normally we use the register of the old copy of input 0 as the target.
-				// However, if input 0 is already in its desired register then we use
-				// the register of the new copy instead.
-				if rp, ok := s.f.getHome(args[0].ID).(*Register); ok {
-					r := register(rp.num)
-					for _, r2 := range dinfo[idx].in[0] {
-						if r == r2 {
-							args[0] = c
-							break
-						}
-					}
-				}
 			}
 
 		ok: