cmd/compile: fix unified IR panic when expanding nested inline function

When reading body of inlining function, which has another inlined
function in the body, the reader still add this inlined function to
todoBodies, which it shouldn't because the inlined function was read
already.

To fix this, introduce new flag to signal that we are done construting
all functions in todoBodies, thus the addBody shouldn't add anything
to todoBodies then.

Updates #48094

Change-Id: I45105dd518f0a7b69c6dcbaf23b957623f271203
Reviewed-on: https://go-review.googlesource.com/c/go/+/347529


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>