[dev.typeparams] cmd/compile: separate out creating instantiations from creating dictionaries

We often need to create a function/method instantiation, but not a
dictionary, because the call to the instantiation will be using a
sub-dictionary. Also, main dictionaries are only need for concrete,
non-gcshape types, whereas instantiations will be for gcshape types (or
concrete types, for strict stenciling).

Created a helper function getDictOrSubdict() to reduce duplicated code.
Also, moved gfGetGfInfo() call in getDictionarySym() inside conditional
where it is needed, to avoid extra work when dictionary has already been
created.

Change-Id: I06587cb2ddc77de2f991e9f9eaf462d2c5a5d45e
Reviewed-on: https://go-review.googlesource.com/c/go/+/332550
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -111,11 +111,6 @@ type gfInfo struct {
 type instInfo struct {
 	fun       *ir.Func // The instantiated function (with body)
 	dictParam *ir.Name // The node inside fun that refers to the dictionary param
-	// Addr of static dictionary associated with this instantiation. This is the
-	// dictionary you should pass if all the type args are concreate. Soon to be
-	// removed, when creating static dictionary and instantiated function are
-	// separated.
-	dictAddr ir.Node
 
 	gf     *ir.Name // The associated generic function
 	gfInfo *gfInfo