cmd/compile: improve error message emitted by debug info generation

Before:

unexpected at 2721:load with unexpected source op v3278unexpected at 2775:load with
unexpected source op v3281unexpected at 2249:load with unexpected source op
v3289unexpected at 2875:load with unexpected source op v3278unexpected at 2232:load
with unexpected source op v286unexpected at 2231:load with unexpected source op
v3291unexpected at 2784:load with unexpected source op v3289unexpected at 2785:load
with unexpected source op v3291

After:

debug info generation: v2721: load with unexpected source op: Phi (v3278)
debug info generation: v2775: load with unexpected source op: Phi (v3281)
debug info generation: v2249: load with unexpected source op: Phi (v3289)
debug info generation: v2875: load with unexpected source op: Phi (v3278)
debug info generation: v2232: load with unexpected source op: Phi (v286)
debug info generation: v2231: load with unexpected source op: Phi (v3291)
debug info generation: v2784: load with unexpected source op: Phi (v3289)
debug info generation: v2785: load with unexpected source op: Phi (v3291)

Updates #25404.

Change-Id: Ib97722848d27ca18bdcd482a610626bc3c6def7d
Reviewed-on: https://go-review.googlesource.com/113275
Run-TryBot: Michael Munday <mike.munday@ibm.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

src/cmd/compile/internal/ssa/debug.go

@@ -157,7 +157,7 @@ type RegisterSet uint64
 // changing this to a Fprintf(os.Stderr) and running make.bash generates
 // thousands of warnings.
 func (s *debugState) unexpected(v *Value, msg string, args ...interface{}) {
-	s.f.Logf("unexpected at "+fmt.Sprint(v.ID)+":"+msg, args...)
+	s.f.Logf("debug info generation: "+fmt.Sprint(v)+": "+msg, args...)
 }
 
 func (s *debugState) logf(msg string, args ...interface{}) {
@@ -477,7 +477,7 @@ func (state *debugState) liveness() []*BlockDebug {
 				case OpStoreReg:
 					source = a.Args[0]
 				default:
-					state.unexpected(v, "load with unexpected source op %v", a)
+					state.unexpected(v, "load with unexpected source op: %v (%v)\n", a.Op, a)
 				}
 			}
 			// Update valueNames with the source so that later steps