[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>

src/cmd/compile/internal/gc/phi.go

@@ -188,6 +188,11 @@ levels:
 			if v.Op == ssa.OpPhi {
 				v.AuxInt = 0
 			}
+			// Any remaining FwdRefs are dead code.
+			if v.Op == ssa.OpFwdRef {
+				v.Op = ssa.OpUnknown
+				v.Aux = nil
+			}
 		}
 	}
 }