cmd/compile/internal/pgo: use a slice for locations
Currently locations are stored in a map and looked up by ID from
the map. The IDs are usually small sequential integers (the Go
pprof CPU profiles are so). Using a slice is more efficient (with
a fallback map to handle weirdly large IDs).
Change-Id: I9e20d5cebca3a5239636413e1bf2f0b273038031
Reviewed-on: https://go-review.googlesource.com/c/go/+/447803
Run-TryBot: Cherry Mui <cherryyz@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>