Pseudo-instructions MOVBS and MOVHS are used to clarify
the semantics of short integers vs. registers:
 * 8-bit and 16-bit values in registers are assumed to always
   be zero-extended or sign-extended depending on their type.
 * MOVB is truncation or move of an already extended value
   between registers.
 * MOVBU enforces zero-extension at the destination (register).
 * MOVBS enforces sign-extension at the destination (register).
And similarly for MOVH/MOVS/MOVHU.

The linker is adapted to assemble MOVB and MOVH to an ordinary
mov. Also a peephole pass in 5g that aims at eliminating
redundant zero/sign extensions is improved.

encoding/binary:
benchmark                              old ns/op    new ns/op    delta
BenchmarkReadSlice1000Int32s              220387       217185   -1.45%
BenchmarkReadStruct                        12839        12910   +0.55%
BenchmarkReadInts                           5692         5534   -2.78%
BenchmarkWriteInts                          6137         6016   -1.97%
BenchmarkPutUvarint32                        257          241   -6.23%
BenchmarkPutUvarint64                        812          754   -7.14%
benchmark                               old MB/s     new MB/s  speedup
BenchmarkReadSlice1000Int32s               18.15        18.42    1.01x
BenchmarkReadStruct                         5.45         5.42    0.99x
BenchmarkReadInts                           5.27         5.42    1.03x
BenchmarkWriteInts                          4.89         4.99    1.02x
BenchmarkPutUvarint32                      15.56        16.57    1.06x
BenchmarkPutUvarint64                       9.85        10.60    1.08x

crypto/des:
benchmark                              old ns/op    new ns/op    delta
BenchmarkEncrypt                            7002         5169  -26.18%
BenchmarkDecrypt                            7015         5195  -25.94%
benchmark                               old MB/s     new MB/s  speedup
BenchmarkEncrypt                            1.14         1.55    1.36x
BenchmarkDecrypt                            1.14         1.54    1.35x

strconv:
benchmark                              old ns/op    new ns/op    delta
BenchmarkAtof64Decimal                       457          385  -15.75%
BenchmarkAtof64Float                         574          479  -16.55%
BenchmarkAtof64FloatExp                     1035          906  -12.46%
BenchmarkAtof64Big                          1793         1457  -18.74%
BenchmarkAtof64RandomBits                   2267         2066   -8.87%
BenchmarkAtof64RandomFloats                 1416         1194  -15.68%
BenchmarkAtof32Decimal                       451          379  -15.96%
BenchmarkAtof32Float                         547          435  -20.48%
BenchmarkAtof32FloatExp                     1095          986   -9.95%
BenchmarkAtof32Random                       1154         1006  -12.82%
BenchmarkAtoi                               1415         1380   -2.47%
BenchmarkAtoiNeg                            1414         1401   -0.92%
BenchmarkAtoi64                             1744         1671   -4.19%
BenchmarkAtoi64Neg                          1737         1662   -4.32%

Fixes #1837.

R=rsc, dave, bradfitz
CC=golang-dev
https://golang.org/cl/12424043