[dev.typeparams] Fix issues related to dictionaries and method calls with embedded fields

 - Fix handling of method expressions with embedded fields. Fix an
   incorrect lookup for method expressions, which have only the
   top-level type (and don't have DOT operations for the embedded
   fields). Add the embedded field dot operations into the closure.

 - Don't need a dictionary and so don't build a closure if the last
   embedded field reached in a method expression is an interface value.

 - Fix methodWrapper() to use the computed 'dot' node in the
   generic-only part of the code.

 - For a method expression, don't create a generic wrapper if the last
   embedded field reached before the method lookup is an interface.

Copied cmd/compile/internal/types2/testdata/fixedbugs/issue44688.go2 to
test/typeparam/issue44688.go, made it fully runnable (rather than just
for compilation), and added a bunch more tests.

Change-Id: I90c1aa569e1c7272e986c9d2ae683e553c3a38a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/329550


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>