cmd/compile: fix reentrancy issue in unified IR function body reading

We shouldn't need to read in function bodies for new functions found
during inlining, but something is expecting them to still be read
in. We should fix that code to not depend on them being read in, but
in the mean time reading them in anyway is at least correct, albeit
less efficient in time and space.

Fixes #49536.
Updates #50552.

Change-Id: I949ef45e7be09406e5a8149e251d78e015aca5fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/390335


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