cmd/compile: reduce overhead of RParams in types.Type

Changed RParams in types.Type to be a pointer to a slice, rather than a
slice itself, in order to reduce it from three words to one words, since
the large majority of types will not be generic or instantiated from a
generic type.

Additional cleanup: remove operation OTYPEINST, which we don't have need
of, since all type instantiations are either handled by types2 or happen
automatically during some form of stenciling.

Both of these are useful cleanups before the Go 1.17 freeze.

Change-Id: I61ad56b3c698b30d2cb5a2cdd12263202c54f49c
Reviewed-on: https://go-review.googlesource.com/c/go/+/308770
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>

src/cmd/compile/internal/ir/node.go

@@ -285,7 +285,6 @@ const (
 	//   Left is nil if there is no type-switch variable
 	OTYPESW
 	OFUNCINST // instantiation of a generic function
-	OTYPEINST // instantiation of a generic type
 
 	// types
 	OTCHAN   // chan int