cmd/compile: fix case for structural types where we should be looking at typeparams

In getInstantiation, we were not computing tparams correctly for the
case where the receiver of a method was a fully-instantiated type. This
wasn't affecting later parts of the function, since method
instantiations of fully-instantiated types were already being calculated
in an earlier path. But it did give us a non-typeparam when trying to
see if a shape was associated with a type param with a structural type.
The fix is just to get the typeparams associated with the base generic
type. Then we can eliminate a conditional check later in the code.
The tparam parameter of Shapify should always be non-nil

Fixes #51367

Change-Id: I6f95fe603886148b2dad0c581416c51373c85009
Reviewed-on: https://go-review.googlesource.com/c/go/+/388116
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

test/typeparam/issue51367.dir/a.go

@@ -0,0 +1,14 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package a
+
+type A[T any] struct{}
+
+func (_ A[T]) Method() {}
+
+func DoSomething[P any]() {
+	a := A[*byte]{}
+	a.Method()
+}

test/typeparam/issue51367.dir/main.go

@@ -0,0 +1,13 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"a"
+)
+
+func main() {
+	a.DoSomething[byte]()
+}

test/typeparam/issue51367.go

@@ -0,0 +1,7 @@
+// rundir -G=3
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ignored