The spec is not conclusive about whether a non-constant shift of
certain untyped constant left operands is valid when the shift
expression appears as an index in an index or slice expression,
or as a size in a `make` function call.

Despite identical spec rules in all these cases, cmd/compile accepts

	make([]byte, 1.0 << s)

but pronounces an error for

	a[1.0 << s]

(go/types accepts both).

This change clarifies the spec by explicitly stating that an
untyped constant left operand in a non-constant shift (1.0 in
the above examples) will be given type `int` in these contexts.

A separate issue #21693 addresses the cmd/compile bug.

Fixes #14844.

Change-Id: I4b52125e487a607fae377fcbed55463cdce9836c
Reviewed-on: https://go-review.googlesource.com/60230Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>