This change replaces

buf := [HUGE_CONST]*T)(unsafe.Pointer(p))[:]

with

buf := [HUGE_CONST]*T)(unsafe.Pointer(p))[:n:n]

Pointer p points to n of T elements. New unsafe pointer conversion
logic verifies that both first and last elements point into the same
Go variable.

This change replaces [:] with [:n:n] to please pointer checker.
According to @mdempsky, compiler specially recognizes when you
combine a pointer conversion with a full slice operation in a single
expression and makes an exception.

After this, only one failure in net remains when running:

go test -a -short -gcflags=all=-d=checkptr std cmd

Updates #34972

Change-Id: I2c8731650c856264bc788e4e07fa0530f7c250fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/208617
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>