cmd/compile: fix miscompilation during inlining

When inlining a function call expression, it's possible that the
function callee subexpression has side effects that need to be
preserved. This used to not be an issue, because inlining wouldn't
recognize these as inlinable anyway. But golang.org/cl/266199 extended
the inlining logic to recognize more cases, but did not notice that
the actual inlining code was discarding side effects.

Issue identified by danscales@.

Fixes #42703.

Change-Id: I95f8fc076b6ca4e9362e80ec26dad9d87a5bc44a
Reviewed-on: https://go-review.googlesource.com/c/go/+/271219


Reviewed-by: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>