cmd/internal/gc: ARM builders broken from optimize slice + write barrier

[bradfitz]

d447279 (cmd/internal/gc: optimize slice + write barrier) broke the ARM builders.

http://build.golang.org/log/25a3d95b7f175ccf03dff342045bce622b6252e7

##### ../test
# go run run.go -- index0.go
exit status 2
# command-line-arguments
/tmp/988792982/tmp__.go:1056: internal compiler error: regalloc 64bit

/cc @minux @josharian

[minux]

A small repro: package main const ci64 int64 = 100014 var si []int = make([]int, 10) func main() { _ = si[0:ci64] }

[gopherbot]

CL https://golang.org/cl/10116 mentions this issue.