cmd/gc: recognize small TPTR64 values as small integer constants.

Given the following Go program:

func sum(s []int) int {
        ret := 0
        for _, x := range s {
                ret += x
        }
        return ret
}

6g would previously generate:

--- prog list "sum" ---
0000 (main.go:3) TEXT    sum+0(SB),$0-24
0001 (main.go:5) MOVQ    s+0(FP),CX
0002 (main.go:5) MOVL    s+8(FP),DI
0003 (main.go:5) MOVL    s+12(FP),BX
0004 (main.go:4) MOVL    $0,SI
0005 (main.go:5) MOVL    $0,AX
0006 (main.go:5) JMP     ,8
0007 (main.go:5) INCL    ,AX
0008 (main.go:5) CMPL    AX,DI
0009 (main.go:5) JGE     $0,16
0010 (main.go:5) MOVL    (CX),DX
0011 (main.go:5) MOVQ    $4,BX
0012 (main.go:5) ADDQ    CX,BX
0013 (main.go:5) MOVQ    BX,CX
0014 (main.go:6) ADDL    DX,SI
0015 (main.go:5) JMP     ,7
0016 (main.go:8) MOVL    SI,.noname+16(FP)
0017 (main.go:8) RET     ,

and now generates:

--- prog list "sum" ---
0000 (main.go:3) TEXT    sum+0(SB),$0-24
0001 (main.go:5) MOVQ    s+0(FP),CX
0002 (main.go:5) MOVL    s+8(FP),DI
0003 (main.go:5) MOVL    s+12(FP),BX
0004 (main.go:4) MOVL    $0,SI
0005 (main.go:5) MOVL    $0,AX
0006 (main.go:5) JMP     ,8
0007 (main.go:5) INCL    ,AX
0008 (main.go:5) CMPL    AX,DI
0009 (main.go:5) JGE     $0,14
0010 (main.go:5) MOVL    (CX),BP
0011 (main.go:5) ADDQ    $4,CX
0012 (main.go:6) ADDL    BP,SI
0013 (main.go:5) JMP     ,7
0014 (main.go:8) MOVL    SI,.noname+16(FP)
0015 (main.go:8) RET     ,

The key difference is that
0011 (main.go:5) MOVQ    $4,BX
0012 (main.go:5) ADDQ    CX,BX
0013 (main.go:5) MOVQ    BX,CX
has changed to
0011 (main.go:5) ADDQ    $4,CX

R=rsc, dave, remyoudompheng
CC=golang-dev
http://codereview.appspot.com/6506089
cmd/gc: Inline pointer sized T2I interface conversions

This CL also adds support for marking the likelyness of IF nodes in the AST being true. This feature is being used here to mark the slow path as unlikely.

src/pkg/runtime:
benchmark                  old ns/op    new ns/op    delta
BenchmarkConvT2IUintptr           16            1  -91.63%

test/bench/go1:
benchmark                 old ns/op    new ns/op    delta
BenchmarkBinaryTree17    5416917000   5461355000   +0.82%
BenchmarkFannkuch11      3810355000   3842609000   +0.85%
BenchmarkGobDecode         19950950     19855420   -0.48%
BenchmarkGobEncode         11301220     11308530   +0.06%
BenchmarkGzip             548119600    546869200   -0.23%
BenchmarkGunzip           176145400    180208300   +2.31%
BenchmarkJSONEncode        93117400     70163100  -24.65%
BenchmarkJSONDecode       406626800    409999200   +0.83%
BenchmarkMandelbrot200      6300992      6317866   +0.27%
BenchmarkParse              7664396      7451625   -2.78%
BenchmarkRevcomp         1189424000   1412332000  +18.74%
BenchmarkTemplate         491308400    458654200   -6.65%

benchmark                  old MB/s     new MB/s  speedup
BenchmarkGobDecode            38.47        38.66    1.00x
BenchmarkGobEncode            67.92        67.87    1.00x
BenchmarkGzip                 35.40        35.48    1.00x
BenchmarkGunzip              110.16       107.68    0.98x
BenchmarkJSONEncode           20.84        27.66    1.33x
BenchmarkJSONDecode            4.77         4.73    0.99x
BenchmarkParse                 7.56         7.77    1.03x
BenchmarkRevcomp             213.69       179.96    0.84x
BenchmarkTemplate              3.95         4.23    1.07x

R=rsc, dave, nigeltao
CC=golang-dev
http://codereview.appspot.com/6351090