During block optimization, degenerate conditional logic such
as "false && x" may result in single-predecessor blocks
containing φ-nodes. (Ideally such φ-nodes would be replaced
by their sole operand, but that requires Referrers information
which isn't computed until later.) It is obviously not safe
to fuse such blocks, so now we don't.
Fixesgolang/go#7840
LGTM=gri
R=gri
CC=golang-codereviews, pcc
https://golang.org/cl/90620043
Robert Griesemer
Alan Donovan