cmd/compile, runtime: stop returning t.zero on hashmap miss

Previously t.zero always pointed to runtime.zerovalue. Change the hashmap code
to always return a runtime pointer directly, and change that pointer to point
to a larger buffer if one is needed.

(It might be better to only copy from the pointer returned by the mapaccess
functions when the value type is small enough and have the compiler insert
explicit zeroing for larger value types, but I tried and failed to do this).

This removes all uses of the zero field of the type data; the field itself can
be removed in a separate change.

Fixes #11491

Change-Id: I5b81752ff4067d74a5a281c41e88f151bae0171e
Reviewed-on: https://go-review.googlesource.com/13784
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/type.go

@@ -257,7 +257,7 @@ type rtype struct {
 	string        *string        // string form; unnecessary but undeniably useful
 	*uncommonType                // (relatively) uncommon fields
 	ptrToThis     *rtype         // type for pointer to this type, if used in binary or has methods
-	zero          unsafe.Pointer // pointer to zero value
+	zero          unsafe.Pointer // unused
 }
 
 // a copy of runtime.typeAlg