There may be further savings if convT2I can avoid the function call
if the cache is good and T is uintptr-shaped, a la convT2E, but that
will be a follow-up CL.

src/pkg/runtime:
benchmark                  old ns/op    new ns/op    delta
BenchmarkConvT2ISmall             43           15  -64.01%
BenchmarkConvT2IUintptr           45           14  -67.48%
BenchmarkConvT2ILarge            130          101  -22.31%

test/bench/go1:
benchmark                 old ns/op    new ns/op    delta
BenchmarkBinaryTree17    8588997000   8499058000   -1.05%
BenchmarkFannkuch11      5300392000   5358093000   +1.09%
BenchmarkGobDecode         30295580     31040190   +2.46%
BenchmarkGobEncode         18102070     17675650   -2.36%
BenchmarkGzip             774191400    771591400   -0.34%
BenchmarkGunzip           245915100    247464100   +0.63%
BenchmarkJSONEncode       123577000    121423050   -1.74%
BenchmarkJSONDecode       451969800    596256200  +31.92%
BenchmarkMandelbrot200     10060050     10072880   +0.13%
BenchmarkParse             10989840     11037710   +0.44%
BenchmarkRevcomp         1782666000   1716864000   -3.69%
BenchmarkTemplate         798286600    723234400   -9.40%

R=rsc, bradfitz, go.peter.90, daniel.morsing, dave, uriel
CC=golang-dev
https://golang.org/cl/6337058