cmd/compile: rename cmd/compile/internal/pgo to cmd/compile/internal/pgoir

cmd/compile: rename cmd/compile/internal/pgo to cmd/compile/internal/pgoir

This helps reduce confusion with cmd/internal/pgo, which performs
compilation-independent analysis. pgoir associates that data with the
IR from the current package compilation.

For #58102.

Change-Id: I9ef1c8bc41db466d3340f41f6d071b95c09566de
Reviewed-on: https://go-review.googlesource.com/c/go/+/569338
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>