cmd/compile: fix ICE when transforming loopvar

When transforming for loop variables, the compiler does roughly
following steps:

	(1) prebody = {z := z' for z in leaked}
        ...
        (4) init' = (init : s/z/z' for z in leaked)

However, the definition of z is not updated to `z := z'` statement,
causing ReassignOracle incorrectly use the new init statement with z'
instead of z, trigger the ICE.

Fixing this by updating the correct/new definition statement for z
during the prebody initialization.

Fixes #73823

Change-Id: Ice2a6741be7478506c58f4000f591d5582029136
Reviewed-on: https://go-review.googlesource.com/c/go/+/675475
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/loopvar/loopvar.go

@@ -305,6 +305,7 @@ func ForCapture(fn *ir.Func) []VarAndLoop {
 						as := ir.NewAssignStmt(x.Pos(), z, tz)
 						as.Def = true
 						as.SetTypecheck(1)
+						z.Defn = as
 						preBody.Append(as)
 						dclFixups[z] = as