Improve performance of highQualityFilter

Move loop invariant caculation out of loop. This gives about 15% speedup
for bitmap_BGRA_8888_A_scale_rotate_bicubic on my desktop i7-3770.

BUG=skia:
R=humper@google.com, mtklein@google.com, reed@google.com

Author: qiankun.miao@intel.com

Review URL: https://codereview.chromium.org/502953004
1 file changed