cmd/compile: avoid dynamic type when possible

cmd/compile: avoid dynamic type when possible

If the expression type is a single compile-time known type, use that
type instead of the dynamic one, so the later passes of the compiler
could skip un-necessary runtime calls.

Thanks Youlin Feng for writing the original test case.

Change-Id: I3f65ab90f041474a9731338a82136c1d394c1773
Reviewed-on: https://go-review.googlesource.com/c/go/+/616975
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>