The following adds support for load and store instructions
with index registers, and adds rules to take advantage of
those instructions.

Examples of improvements:

crypto/rc4:
name     old time/op   new time/op   delta
RC4_128    445ns ± 0%    404ns ± 0%   -9.21%  (p=0.029 n=4+4)
RC4_1K    3.46µs ± 0%   3.13µs ± 0%   -9.29%  (p=0.029 n=4+4)
RC4_8K    27.0µs ± 0%   24.7µs ± 0%   -8.83%  (p=0.029 n=4+4)

crypto/des:
name         old time/op    new time/op    delta
Encrypt         276ns ± 0%     264ns ± 0%  -4.35%  (p=0.029 n=4+4)
Decrypt         278ns ± 0%     263ns ± 0%  -5.40%  (p=0.029 n=4+4)
TDESEncrypt     683ns ± 0%     645ns ± 0%  -5.56%  (p=0.029 n=4+4)
TDESDecrypt     684ns ± 0%     641ns ± 0%  -6.29%  (p=0.029 n=4+4)

crypto/sha1:
name          old time/op    new time/op    delta
Hash8Bytes       661ns ± 0%     635ns ± 0%  -3.93%  (p=1.000 n=1+1)
Hash320Bytes    2.70µs ± 0%    2.56µs ± 0%  -5.26%  (p=1.000 n=1+1)
Hash1K          7.14µs ± 0%    6.78µs ± 0%  -5.03%  (p=1.000 n=1+1)
Hash8K          52.1µs ± 0%    49.4µs ± 0%  -5.14%  (p=1.000 n=1+1)

Change-Id: I03810e90fcc20029975a323f06bfa086c973c2b0
Reviewed-on: https://go-review.googlesource.com/c/135975
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>