runtime: on map update, don't overwrite key if we don't need to.

Keep track of which types of keys need an update and which don't.

Strings need an update because the new key might pin a smaller backing store.
Floats need an update because it might be +0/-0.
Interfaces need an update because they may contain strings or floats.

Fixes #11088

Change-Id: I9ade53c1dfb3c1a2870d68d07201bc8128e9f217
Reviewed-on: https://go-review.googlesource.com/10843
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/type.go

@@ -67,6 +67,7 @@ type maptype struct {
 	indirectvalue bool   // store ptr to value instead of value itself
 	bucketsize    uint16 // size of bucket
 	reflexivekey  bool   // true if k==k for all keys
+	needkeyupdate bool   // true if we need to update key on an overwrite
 }
 
 type chantype struct {