cmd/compile: eliminate stores to unread auto variables

This is a crude compiler pass to eliminate stores to auto variables
that are only ever written to.

Eliminates an unnecessary store to x from the following code:

func f() int {
	var x := 1
	return *(&x)
}

Fixes #19765.

Change-Id: If2c63a8ae67b8c590b6e0cc98a9610939a3eeffa
Reviewed-on: https://go-review.googlesource.com/38746
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/runtime-gdb_test.go

@@ -84,7 +84,7 @@ func main() {
 	slicevar := make([]string, 0, 16)
 	slicevar = append(slicevar, mapvar["abc"])
 	fmt.Println("hi") // line 13
-	_ = ptrvar
+	runtime.KeepAlive(ptrvar)
 	gslice = slicevar
 	runtime.KeepAlive(mapvar)
 }