Remove the unnecessary eta abstraction reported in golang/go#65217, by
introducing a new strategy for rewriting assignments.

This strategy involves analyzing both LHS and RHS of an assignment, and
choosing between three substrategies:
 - spread the result expressions in cases where types are unambiguous
 - predeclare LHS variables in cases where the return is itself a spread
   call
 - convert RHS expressions if types involve implicit conversions

Doing this involved some fixes to the logic for detecting trivial
conversions, and tracking some additional information about untyped nils
in return expressions.

Since this strategy avoids literalization by modifying assignments in
place, it must be able to avoid nested blocks, and so it explicitly
records that braces may be elided.

There is more work to be done here, both improving the writeType helper,
and by variable declarations, but this CL lays a foundation for later
expansion.

For golang/go#65217

Change-Id: I9b3b595f7f678ab9b86ef7cf19936fd818b45426
Reviewed-on: https://go-review.googlesource.com/c/tools/+/580835
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>