A new node type OSPTR is added to refer to the data pointer of
strings and slices in a simple way during walk(). It will be
useful for future work on simplification of slice arithmetic.

benchmark                  old ns/op    new ns/op    delta
BenchmarkCopy1Byte                 9            8  -13.98%
BenchmarkCopy2Byte                14            8  -40.49%
BenchmarkCopy4Byte                13            8  -35.04%
BenchmarkCopy8Byte                13            8  -37.10%
BenchmarkCopy12Byte               14           12  -15.38%
BenchmarkCopy16Byte               14           12  -17.24%
BenchmarkCopy32Byte               19           14  -27.32%
BenchmarkCopy128Byte              31           26  -15.29%
BenchmarkCopy1024Byte            100           92   -7.50%
BenchmarkCopy1String              10            7  -28.99%
BenchmarkCopy2String              10            7  -28.06%
BenchmarkCopy4String              10            8  -22.69%
BenchmarkCopy8String              10            8  -23.30%
BenchmarkCopy12String             11           11   -5.88%
BenchmarkCopy16String             11           11   -5.08%
BenchmarkCopy32String             15           14   -6.58%
BenchmarkCopy128String            28           25  -10.60%
BenchmarkCopy1024String           95           95   +0.53%

R=golang-dev, bradfitz, cshapiro, dave, daniel.morsing, rsc, khr, khr
CC=golang-dev
https://golang.org/cl/9101048