cmd/compile: ensure we use allowed registers for input-clobbering instructions

For instructions which clobber their input register, we make a second
copy of the input value so it is still available in a register for
future instructions.

That second copy might not respect the register input restrictions
for the instruction. So the second copy we make here can't actually
be used by the instruction - it should use the first copy, the second
copy is the one that will persist beyond the clobber.

Fixes #75063

Change-Id: I99acdc63f0c4e54567a174ff7ada601ae4e796b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/697015
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -1725,10 +1725,9 @@ func (s *regAllocState) regalloc(f *Func) {
 					// spilling the value with the most distant next use.
 					continue
 				}
-				// Copy input to a new clobberable register.
+				// Copy input to a different register that won't be clobbered.
 				c := s.allocValToReg(v.Args[i], m, true, v.Pos)
 				s.copies[c] = false
-				args[i] = c
 			}
 
 			// Pick a temporary register if needed.

test/fixedbugs/issue75063.go

@@ -0,0 +1,75 @@
+// compile
+
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reorder
+
+type Element struct {
+	A     string
+	B     string
+	C     string
+	D     string
+	E     string
+	Text  []string
+	List  []string
+	Child Elements
+	F     string
+	G     bool
+	H     bool
+	I     string
+}
+
+type Elements []Element
+
+func DoesNotCompile(ve Elements) Elements {
+	aa := Elements{}
+	bb := Elements{}
+	cc := Elements{}
+	dd := Elements{}
+	ee := Elements{}
+	ff := Elements{}
+	gg := Elements{}
+	hh := Elements{}
+	ii := Elements{}
+
+	if len(ve) != 1 {
+		return ve
+	}
+	for _, e := range ve[0].Child {
+		if len(e.Text) == 1 && (e.Text[0] == "xx") {
+			ee = append(ee, e)
+		} else if len(e.Text) == 1 && e.Text[0] == "yy" {
+			for _, c := range e.Child {
+				if len(c.Text) == 1 && c.Text[0] == "zz" {
+					ii = append(ii, c)
+				} else {
+					hh = append(hh, c)
+				}
+			}
+			ii = append(ii, hh...)
+			e.Child = ii
+			gg = append(gg, e)
+		} else if len(e.Text) == 1 && e.Text[0] == "tt" {
+			for _, entry := range e.Child {
+				for _, c := range entry.Child {
+					if len(c.Text) == 1 && c.Text[0] == "ee" {
+						cc = append(cc, c)
+					} else {
+						dd = append(dd, c)
+					}
+				}
+				cc = append(cc, dd...)
+				entry.Child = cc
+				bb = append(bb, entry)
+				cc, dd = Elements{}, Elements{}
+			}
+			e.Child = bb
+			aa = append(aa, e)
+		} else {
+			ff = append(ff, e)
+		}
+	}
+	return ve
+}