cmd/compile: make prove use non-equality in subtraction for a stronger bound

[uellenberg]

Given:
s := /* slice /
k := / proved valid index in s (0 <= k < len(s)) */
v := s[k:]

len(v) >= 1, so v[0] needs no bounds check. However, for
len(v) = len(s) - k, we only checked if len(s) >= k and so could only
prove len(v) >= 0, thus the bounds check wasn't removed.

As far as I can tell these checks were commented out for performance,
but after benchmarking prove I see no difference.

Fixes: #76429

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