cmd/compile: break out transformations of tcCompLit into transformCompLit

Create transformCompLit, which does the transformations done by
tcCompLit without the typechecking. This removes the final use of the
old typechecker in the noder2 pass.

Other changes:

 - Used the transformCompLit in stringstorunelit(), which creates an
   OCOMPLIT that needs transformation as well.

 - Fixed one place in transformIndex where we were still using
   typecheck.AssignConv, when we should be using its equivalent
   noder.assignconvfn.

The go/test tests always run with -G=3, and I also tested that the "go
test" tests continue to run correctly with -G=3.

Change-Id: I4a976534ab7311cf2a5f43841026dbf7401e62b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/308529


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>