cmd/compile: avoid re-instantiating method that is already imported

We can import an shape-instantiated function/method for inlining
purposes. If we are instantiating the methods of a instantiated type
that we have seen, and it happens to need a shape instantiation that we
have imported, then don't re-create the instantiation, since we will end
up with conflicting/duplicate definitions for the instantiation symbol.
Instead, we can just use the existing imported instantation, and enter
it in the instInfoMap[].

Fixes #50121

Change-Id: I6eeb8786faad71106e261e113048b579afad04fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/371414


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>