cmd/compile: preserve pointerness when creating map key temp

When creating the temporary for map functions, if the key
contains pointer, we need to create pointer-typed temporary. So
if the temporary is live across a function call, the pointer is
live.

Change-Id: Id6e14ec9def8bc7987f0f8ce8423caf1e3754fcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/311379


Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>