cmd/compile: always zero the temporary in mapKeyTemp

As we are taking its address, always zero it. In many cases the
temporary will be optimized out. But in case it does not (e.g. -N,
-race), this ensures it has the right liveness information.

May fix the noopt builder.

Change-Id: I3d5d617c276d2a1a1aaebff813b4cd60bc691592
Reviewed-on: https://go-review.googlesource.com/c/go/+/309771


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>