LibJS: Weekend optimization pack

[awesomekling]

Another batch of small optimizations.
Roughly 3% speed-up across all of Octane while Kraken looks neutral.

Suite       Test                                   Speedup  Old (Mean ± Range)           New (Mean ± Range)
----------  -----------------------------------  ---------  ---------------------------  --------------------------
Kraken      ai-astar.js                              1.011  2.710 ± 2.700 … 2.720        2.680 ± 2.660 … 2.700
Kraken      audio-beat-detection.js                  0.994  1.770 ± 1.760 … 1.780        1.780 ± 1.780 … 1.780
Kraken      audio-dft.js                             1.008  1.340 ± 1.340 … 1.340        1.330 ± 1.310 … 1.350
Kraken      audio-fft.js                             0.997  1.670 ± 1.670 … 1.670        1.675 ± 1.670 … 1.680
Kraken      audio-oscillator.js                      0.984  1.570 ± 1.570 … 1.570        1.595 ± 1.590 … 1.600
Kraken      imaging-darkroom.js                      1.005  2.180 ± 2.180 … 2.180        2.170 ± 2.070 … 2.270
Kraken      imaging-desaturate.js                    0.934  2.345 ± 2.340 … 2.350        2.510 ± 2.500 … 2.520
Kraken      imaging-gaussian-blur.js                 1.006  8.965 ± 8.960 … 8.970        8.910 ± 8.570 … 9.250
Kraken      json-parse-financial.js                  1.04   0.130 ± 0.130 … 0.130        0.125 ± 0.120 … 0.130
Kraken      json-stringify-tinderbox.js              0.98   0.240 ± 0.240 … 0.240        0.245 ± 0.240 … 0.250
Kraken      stanford-crypto-aes.js                   1.022  0.705 ± 0.700 … 0.710        0.690 ± 0.680 … 0.700
Kraken      stanford-crypto-ccm.js                   1.009  0.590 ± 0.590 … 0.590        0.585 ± 0.580 … 0.590
Kraken      stanford-crypto-pbkdf2.js                0.996  1.165 ± 1.160 … 1.170        1.170 ± 1.150 … 1.190
Kraken      stanford-crypto-sha256-iterative.js      1.054  0.485 ± 0.480 … 0.490        0.460 ± 0.450 … 0.470
Octane      box2d.js                                 1.012  2.460 ± 2.450 … 2.470        2.430 ± 2.420 … 2.440
Octane      code-load.js                             0.995  2.155 ± 2.150 … 2.160        2.165 ± 2.160 … 2.170
Octane      crypto.js                                1.002  7.250 ± 7.230 … 7.270        7.235 ± 7.130 … 7.340
Octane      deltablue.js                             1      2.035 ± 2.010 … 2.060        2.035 ± 2.030 … 2.040
Octane      earley-boyer.js                          0.999  17.470 ± 17.280 … 17.660     17.480 ± 17.390 … 17.570
Octane      gbemu.js                                 1.032  3.220 ± 3.210 … 3.230        3.120 ± 3.090 … 3.150
Octane      mandreel.js                              1.022  15.030 ± 14.570 … 15.490     14.700 ± 14.490 … 14.910
Octane      navier-stokes.js                         1.03   3.215 ± 3.210 … 3.220        3.120 ± 3.110 … 3.130
Octane      pdfjs.js                                 1.002  3.005 ± 3.000 … 3.010        3.000 ± 2.990 … 3.010
Octane      raytrace.js                              1.072  7.125 ± 7.060 … 7.190        6.645 ± 6.500 … 6.790
Octane      regexp.js                                0.997  22.210 ± 22.140 … 22.280     22.285 ± 22.150 … 22.420
Octane      richards.js                              1.005  2.020 ± 2.020 … 2.020        2.010 ± 2.010 … 2.010
Octane      splay.js                                 1.029  3.045 ± 3.040 … 3.050        2.960 ± 2.910 … 3.010
Octane      typescript.js                            0.984  37.695 ± 37.640 … 37.750     38.300 ± 38.180 … 38.420
Octane      zlib.js                                  1.066  106.620 ± 105.740 … 107.500  100.040 ± 99.850 … 100.230
Kraken      Total                                    0.998  25.865                       25.925
Octane      Total                                    1.031  234.555                      227.525
All Suites  Total                                    1.028  260.420                      253.450