gopls/internal/analysis/gofix: one function per pass

Split the two passes into two separate functions.
Declare a type to hold common state.

No behavior changes.

For golang/go#32816.

Change-Id: I571956859b12687d824f36c80f75deb96db38d92
Reviewed-on: https://go-review.googlesource.com/c/tools/+/649476
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: jba