This adds new rules to recognize consecutive byte loads and
stores and lowers them to loads and stores such as lhz, lwz, ld,
sth, stw, std. This change only covers the little endian cases
on little endian machines, such as is found in encoding/binary
UintXX or PutUintXX for little endian. Big endian will be done
later.

Updates were also made to binary_test.go to allow the benchmark
for Uint and PutUint to actually use those functions because
the way they were written, those functions were being
optimized out.

Testcases were also added to cmd/compile/internal/gc/asm_test.go.

Updates #22496

The following improvement can be found in golang.org/x/crypto

poly1305:

Benchmark64-16              142           114           -19.72%
Benchmark1K-16              1717          1424          -17.06%
Benchmark64Unaligned-16     142           113           -20.42%
Benchmark1KUnaligned-16     1721          1428          -17.02%

chacha20poly1305:

BenchmarkChacha20Poly1305Open_64-16     1012       885   -12.55%
BenchmarkChacha20Poly1305Seal_64-16     971        836   -13.90%
BenchmarkChacha20Poly1305Open_1350-16   11113      9539  -14.16%
BenchmarkChacha20Poly1305Seal_1350-16   11013      9392  -14.72%
BenchmarkChacha20Poly1305Open_8K-16     61074      53431 -12.51%
BenchmarkChacha20Poly1305Seal_8K-16     61214      54806 -10.47%

Other improvements of around 10% found in crypto/tls.

Results after updating encoding/binary/binary_test.go:

BenchmarkLittleEndianPutUint64-16     1.87      0.93      -50.27%
BenchmarkLittleEndianPutUint32-16     1.19      0.93      -21.85%
BenchmarkLittleEndianPutUint16-16     1.16      1.03      -11.21%

Change-Id: I7bbe2fbcbd11362d58662fecd907a0c07e6ca2fb
Reviewed-on: https://go-review.googlesource.com/74410
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <mike.munday@ibm.com>