Optimize add operation.

[zherczeg]

Improves 3d-cube.js and math-spectral-norm.js

Benchmark	Perf (sec)
3d-cube.js	1.482 -> 1.308 : +11.718%
3d-raytrace.js	1.584 -> 1.539 : +2.815%
access-binary-trees.js	0.749 -> 0.744 : +0.615%
access-fannkuch.js	3.100 -> 3.124 : -0.752%
access-nbody.js	1.558 -> 1.525 : +2.089%
bitops-3bit-bits-in-byte.js	0.792 -> 0.751 : +5.165%
bitops-bits-in-byte.js	1.041 -> 1.048 : -0.725%
bitops-bitwise-and.js	1.875 -> 1.973 : -5.222%
bitops-nsieve-bits.js	2.523 -> 2.475 : +1.901%
controlflow-recursive.js	0.549 -> 0.542 : +1.173%
crypto-aes.js	1.445 -> 1.449 : -0.280%
crypto-md5.js	0.934 -> 0.899 : +3.746%
crypto-sha1.js	0.880 -> 0.849 : +3.485%
date-format-tofte.js	1.140 -> 1.145 : -0.405%
date-format-xparb.js	0.591 -> 0.578 : +2.265%
math-cordic.js	2.002 -> 1.887 : +5.747%
math-partial-sums.js	1.096 -> 1.058 : +3.468%
math-spectral-norm.js	0.892 -> 0.789 : +11.580%
string-base64.js	4.692 -> 4.647 : +0.960%
string-fasta.js	2.241 -> 2.188 : +2.363%
Geometric mean:	+2.663%

[LaszloLango]

LGTM

[dbatyai]

LGTM