Open the error console (via the Web Developer menu item) and you'll see messages there. Success shows up as "Error: successfully compiled asm.js code." :-)
P.S. Looking at your code, I can tell it's not asm.js conformant yet, though it should just be a bunch of small tweaks (e.g. an expression like (a+b) will need to be (a+b)|0, etc.).
Can you tell me, how did you confirm you wrote valid asm.js and there was no fallback-mechanism kicking in?
I benchmarked my implementation, of course, and noticed a 2x speedup. However, this seems quite less for the this type of low-level algorithm...