The language spec requires the RHS operand of shift expressions to be unsigned integers.

The changes in CL 60230 and the related CL 81277 refer to a variable s of type uint.
The "untyped constant" here refers to 1.0, not s.

Change-Id: Id2b884816af7f79f453afcb8c34ade2d34e18bc2
GitHub-Last-Rev: b26c853cae2adea7235a51ad726308e337494385
GitHub-Pull-Request: golang/go#24989
Reviewed-on: https://go-review.googlesource.com/108676Reviewed-by: Robert Griesemer <gri@golang.org>