cmd/gc: don't mark select descriptor as EscNone

selv is created with temp() which calls tempname, which marks
the new n with EscNever, so there is no need to explicitly set
EscNone on the select descriptor.

Fixes #8396.

LGTM=dvyukov
R=golang-codereviews, dave, dvyukov
CC=golang-codereviews
https://golang.org/cl/112520043

src/cmd/gc/select.c

@@ -260,7 +260,6 @@ walkselect(Node *sel)
 	// generate sel-struct
 	setlineno(sel);
 	selv = temp(selecttype(sel->xoffset));
-	selv->esc = EscNone;
 	r = nod(OAS, selv, N);
 	typecheck(&r, Etop);
 	init = list(init, r);