cmd/compile: use generics for sparse map

cmd/compile: use generics for sparse map

So it is easier to reuse this code with different key/value types.

Change-Id: I5a9e669769cf359b32f2fe784594868acdee4d02
Reviewed-on: https://go-review.googlesource.com/c/go/+/681175
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>