cmd/gc: distinguish unnamed vs blank-named return variables better

Before, an unnamed return value turned into an ONAME node n with n->sym
named ~anon%d, and n->orig == n.

A blank-named return value turned into an ONAME node n with n->sym
named ~anon%d but n->orig == the original blank n. Code generation and
printing uses n->orig, so that this node formatted as _.

But some code does not use n->orig. In particular the liveness code does
not know about the n->orig convention and so mishandles blank identifiers.
It is possible to fix but seemed better to avoid the confusion entirely.

Now the first kind of node is named ~r%d and the second ~b%d; both have
n->orig == n, so that it doesn't matter whether code uses n or n->orig.

After this change the ->orig field is only used for other kinds of expressions,
not for ONAME nodes.

This requires distinguishing ~b from ~r names in a few places that care.
It fixes a liveness analysis bug without actually changing the liveness code.

TBR=ken2
CC=golang-codereviews
https://golang.org/cl/63630043

test/live.go

@@ -79,3 +79,10 @@ func f5(b1 bool) {
 	}
 	print(**z) // ERROR "live at call to printint: x y$"
 }
+
+// confusion about the _ result used to cause spurious "live at entry to f6: _".
+
+func f6() (_, y string) {
+	y = "hello"
+	return
+}