Also adjust truncfltlit to make it more similar to trunccmplxlit, and
make it report an error for bad Etypes.

Fixes #19947

Change-Id: I6684523e989c2293b8a8e85bd2bfb9c399c5ea36
Reviewed-on: https://go-review.googlesource.com/40453Reviewed-by: Robert Griesemer <gri@golang.org>