cmd/compile: "VARDEF is not a top level statement" compiler crash on valid program

[ALTree]

$ gotip version
go version devel +8aee193fb8 Sat Oct 6 12:11:53 2018 +0000 linux/amd64

package p

func f() {
	var s string
	var as []string
	switch false && (s+"a"+as[0]+s+as[0]+s == "") {
	}
}

$ gotip build crash.go
# command-line-arguments
nottop [0xc0004cae00]
.   VARDEF l(6)
.   .   NAME-p..autotmp_2 a(true) l(6) x(0) class(PAUTO) esc(N) tc(1) addrtaken used SLICE-[6]string
./crash.go:6:21: VARDEF is not a top level statement

It compiles fine on go1.11

[ALTree]

Git bisect points to 9a8372f (cmd/compile,runtime: remove ambiguously live logic) as the first bad commit.

cc @randall77

[gopherbot]

Change https://golang.org/cl/140304 mentions this issue: cmd/compile: allow VARDEF at top level