[dev.ssa] cmd/compile/internal/ssa: minor fixes

* Improve some docs and logging.
* Set correct type and len for indexing into strings.

Fixes #11029.

Change-Id: Ib22c45908e41ba3752010d2f5759e37e3921a48e
Reviewed-on: https://go-review.googlesource.com/10635
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -82,7 +82,7 @@ func deadcode(f *Func) {
 			i++
 		} else {
 			if len(b.Values) > 0 {
-				panic("live value in unreachable block")
+				log.Panicf("live values in unreachable block %v: %v", b, b.Values)
 			}
 			f.bid.put(b.ID)
 		}