cmd/compile: add debug log prior to export data lookup

If there is a crash in LookupFunc (which has occurred a few times now),
this ensures that we log the offending symbol before crashing.

For #67016.

Change-Id: I0119597de2be3d1b97c41a9361273d1feb90ec11
Reviewed-on: https://go-review.googlesource.com/c/go/+/581437
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: prattmic

src/cmd/compile/internal/pgoir/irgraph.go

@@ -333,6 +333,9 @@ func addIndirectEdges(g *IRGraph, namedEdgeMap pgo.NamedEdgeMap) {
 			// devirtualization. Instantiation of generic functions
 			// will likely need to be done at the devirtualization
 			// site, if at all.
+			if base.Debug.PGODebug >= 3 {
+				fmt.Printf("addIndirectEdges: %s attempting export data lookup\n", key.CalleeName)
+			}
 			fn, err := LookupFunc(key.CalleeName)
 			if err == nil {
 				if base.Debug.PGODebug >= 3 {