cmd/compile/internal/pgo: remove most global state

Since pgo is a new package, it is reasonably straightforward to
encapsulate its state into a non-global object that we pass around,
which will help keep it isolated.

There are no functional changes in this CL, just packaging up the
globals into a new object.

There are two major pieces of cleanup remaining:

1. reflectdata and noder have separate InlineCalls calls for method
   wrappers. The Profile is not plumbed there yet, but this is not a
   regression as the globals were previously set only right around the
   main inlining pass in gc.Main.

2. pgo.ListOfHotCallSites is still global, as it will require more work
   to clean up. It is effectively a local variable in InlinePackage,
   except that it assumes that InlineCalls is immediately preceded by a
   CanInline call for the same function. This is not necessarily true
   due to the recursive nature of CanInline. This also means that some
   InlineCalls calls may be missing the list of hot callsites right now.

For #55022.

Change-Id: Ic1fe41f73df96861c65f8bfeecff89862b367290
Reviewed-on: https://go-review.googlesource.com/c/go/+/446303
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Pratt <mpratt@google.com>

src/cmd/compile/internal/noder/reader.go

@@ -3867,7 +3867,8 @@ func finishWrapperFunc(fn *ir.Func, target *ir.Package) {
 
 	// We generate wrappers after the global inlining pass,
 	// so we're responsible for applying inlining ourselves here.
-	inline.InlineCalls(fn)
+	// TODO(prattmic): plumb PGO.
+	inline.InlineCalls(fn, nil)
 
 	// The body of wrapper function after inlining may reveal new ir.OMETHVALUE node,
 	// we don't know whether wrapper function has been generated for it or not, so