cmd/compile/internal/ssa: expand runtime.memequal for length {3,5,6,7}

cmd/compile/internal/ssa: expand runtime.memequal for length {3,5,6,7}

This CL slightly speeds up strings.HasPrefix when testing constant
prefixes of length {3,5,6,7}.

goos: linux
goarch: amd64
cpu: Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
                │      old     │                 new                 │
                │    sec/op    │   sec/op     vs base                │
StringPrefix3-8   11.125n ± 2%   8.539n ± 1%  -23.25% (p=0.000 n=20)
StringPrefix5-8   11.170n ± 2%   8.700n ± 1%  -22.11% (p=0.000 n=20)
StringPrefix6-8   11.190n ± 2%   8.655n ± 1%  -22.65% (p=0.000 n=20)
StringPrefix7-8   11.095n ± 1%   8.878n ± 1%  -19.98% (p=0.000 n=20)

Change-Id: I510a80d59cf78680b57d68780d35d212d24030e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/700816
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Keith Randall <khr@golang.org>