Disable at-simd warning.

[yuyichao]

It's not helpful/annoy for users because,

1. It doesn't cover the majority of the cases when a loop can't be vectorized (the majority of the cases that matters anyway).
2. It makes it hard to put @simd on a loop that might be vectorizable without getting the warning for non builtin types.

It's not really helpful for debugging vectorization failure due to codegen issues either since

1. In it's current state (probably different before), I've never seen a single case captured by it
2. It doesn't print any useful debug info with it so you need some other ways to print the llvm ir anyway

[yuyichao]

I believe the vectorization label is for the matlab-style vectorized array operation, not for vector instruction/loop vectorization.

[ArchRobison]

@yuyichao makes a convincing case -- the patch looks good to me.

[StefanKarpinski]

Should we backport this to 0.5.1? This won't break code in either direction since it's a warning.