cmd/compile: add Trunc-of-Ext simplifications

cmd/compile: add Trunc-of-Ext simplifications

This is a follow-up to the discussion
in CL 27853.

During make.bash, trigger count:

24 rewrite generic.rules:57
22 rewrite generic.rules:69
10 rewrite generic.rules:54
10 rewrite generic.rules:58
10 rewrite generic.rules:67
 7 rewrite generic.rules:66
 4 rewrite generic.rules:59
 3 rewrite generic.rules:50
 3 rewrite generic.rules:51
 3 rewrite generic.rules:52
 1 rewrite generic.rules:64

Change-Id: Id96cb6a707a4a564831f763c2d4d0e180c94bbef
Reviewed-on: https://go-review.googlesource.com/28088
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Martin Möhrmann <martisch@uos.de>