compiler, runtime: panic on uncomparable map key, even if map is empty

This ports https://golang.org/cl/155918 from the master repo.

    runtime: panic on uncomparable map key, even if map is empty

    Reorg map flags a bit so we don't need any extra space for the extra flag.

This is a pre-req for updating libgo to the Go 1.12beta2 release.

Change-Id: I31dc787bb2a29a8d991a9b119eeaa4fea00fad30
Reviewed-on: https://go-review.googlesource.com/c/157858
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
5 files changed