cmd/compile/internal/ssa: generate bswap on AMD64

Generate bswap+load/store for reading/writing big endian data.
Helps encoding/binary.

name                    old time/op    new time/op    delta
ReadSlice1000Int32s-8     5.06µs ± 8%    4.58µs ± 8%   -9.50%        (p=0.000 n=10+10)
ReadStruct-8              1.07µs ± 0%    1.05µs ± 0%   -1.51%         (p=0.000 n=9+10)
ReadInts-8                 367ns ± 0%     363ns ± 0%   -1.15%          (p=0.000 n=8+9)
WriteInts-8                475ns ± 1%     469ns ± 0%   -1.45%        (p=0.000 n=10+10)
WriteSlice1000Int32s-8    5.03µs ± 3%    4.50µs ± 3%  -10.45%          (p=0.000 n=9+9)
PutUvarint32-8            17.2ns ± 0%    17.2ns ± 0%     ~     (all samples are equal)
PutUvarint64-8            46.7ns ± 0%    46.7ns ± 0%     ~           (p=0.509 n=10+10)

name                    old speed      new speed      delta
ReadSlice1000Int32s-8    791MB/s ± 8%   875MB/s ± 8%  +10.53%        (p=0.000 n=10+10)
ReadStruct-8            70.0MB/s ± 0%  71.1MB/s ± 0%   +1.54%         (p=0.000 n=9+10)
ReadInts-8              81.6MB/s ± 0%  82.6MB/s ± 0%   +1.21%          (p=0.000 n=9+9)
WriteInts-8             63.0MB/s ± 1%  63.9MB/s ± 0%   +1.45%        (p=0.000 n=10+10)
WriteSlice1000Int32s-8   796MB/s ± 4%   888MB/s ± 3%  +11.65%          (p=0.000 n=9+9)
PutUvarint32-8           233MB/s ± 0%   233MB/s ± 0%     ~           (p=0.089 n=10+10)
PutUvarint64-8           171MB/s ± 0%   171MB/s ± 0%     ~            (p=0.137 n=10+9)

Change-Id: Ia2dbdef92198eaa7e2af5443a8ed586d4b401ffb
Reviewed-on: https://go-review.googlesource.com/32222


Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>