slices: use !{{Less}} instead of {{GreaterOrEqual}}
    
    In CL 371574 PatchSet 18, we replaced all !{{Less}} with {{GreaterOrEqual}} to fix a problem(handle NaNs when sorting float64 slice) in exp/slices.
    
    We don't actually need this change, because we don't guarantee that the slice will be sorted eventually if there are NaNs(we could have a[i] < a[j] for some i,j with i>j).
    
    This CL reverts all the replacements in exp/slices and does not affect any codes in the sort package.
    
    Change-Id: Idc225d480de3e2efef2add35c709ed880d1306cb
    Reviewed-on: https://go-review.googlesource.com/c/go/+/400534
    Reviewed-by: Keith Randall <khr@golang.org>
    Run-TryBot: Keith Randall <khr@golang.org>
    Auto-Submit: Keith Randall <khr@golang.org>
    TryBot-Result: Gopher Robot <gobot@golang.org>
    Reviewed-by: Eli Bendersky <eliben@google.com>
    Reviewed-by: Keith Randall <khr@google.com>
    Auto-Submit: Keith Randall <khr@google.com>