Use uint index variables in some cases instead
of int to enable strength reduction; this makes
it possible for the compiler to reduce % into
masks.

Old code: 6g -S md4.go md4block.go | grep "md4block.go:44"
0471 (md4block.go:44) MOVL    AX,BX
0472 (md4block.go:44) MOVL    AX,BP
0473 (md4block.go:44) MOVL    AX,R8
0474 (md4block.go:44) SARL    $31,R8
0475 (md4block.go:44) SHRL    $30,R8
0476 (md4block.go:44) ADDL    R8,BP
0477 (md4block.go:44) SARL    $2,BP
0478 (md4block.go:44) IMULL   $4,BP
0479 (md4block.go:44) SUBL    BP,BX
0480 (md4block.go:44) MOVLQSX BX,BX
0481 (md4block.go:44) LEAQ    shift1+0(SB),BP
0482 (md4block.go:44) CMPL    BX,8(BP)
0483 (md4block.go:44) JCS     ,485
0484 (md4block.go:44) CALL    ,runtime.throwindex+0(SB)
0485 (md4block.go:44) MOVQ    (BP),BP
0486 (md4block.go:44) MOVL    (BP)(BX*4),DI

New code: 6g -S md4.go md4block.go | grep "md4block.go:44"
0471 (md4block.go:44) MOVL    AX,BX
0472 (md4block.go:44) ANDL    $3,BX
0473 (md4block.go:44) MOVLQZX BX,BX
0474 (md4block.go:44) LEAQ    shift1+0(SB),BP
0475 (md4block.go:44) CMPL    BX,8(BP)
0476 (md4block.go:44) JCS     ,478
0477 (md4block.go:44) CALL    ,runtime.throwindex+0(SB)
0478 (md4block.go:44) MOVQ    (BP),BP
0479 (md4block.go:44) MOVL    (BP)(BX*4),DI

R=agl, agl1
CC=golang-dev
https://golang.org/cl/181086