[dev.regabi] cmd/compile: fix noopt builder

The non-simple, phi-insertion algorithm can leave OpFwdRefs in the SSA
graph unresolved if they're in dead blocks. Normally, these would be
harmlessly removed later during SSA dead-code elimination, but those
passes are omitted for -N builds. And so they reach zcse, where the
Value.Aux is used within a hash map.

This became a problem after golang.org/cl/275788, which added
FwdRefAux to wrap OpFwdRef's ir.Node, and to ensure that it's not
compared for equality / used as a map key.

This CL adds a simple fix: if there are any OpFwdRefs remaining after
resolveFwdRef, then they must be dead code and we can simply replace
them with OpUnknown.

Change-Id: I72e4116d52d3f6441ebb0bf6160906617cd59513
Reviewed-on: https://go-review.googlesource.com/c/go/+/277075


Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>