cmd/compile: fix ICE due to missing inline function body

cmd/compile: fix ICE due to missing inline function body

For golang.org/cl/74110, I forgot that you can use range-based for
loops to extract key values from a map value.

This wasn't a problem for the binary format importer, because it was
more tolerant about missing inline function bodies. However, the
indexed importer is more particular about this.

We could potentially just make it more lenient like the binary
importer, but tweaking the logic here is easy enough and seems like
the preferable solution.

Fixes #26341.

Change-Id: I54564dcd0be60ea393f8a0f6954b7d3d61e96ee5
Reviewed-on: https://go-review.googlesource.com/123475
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>