Texture functions that are in a divergent path can still take a bias parameter.
Treat the bias parameter as an explicit lod. The logic behind this is that the
implicit lod is considerd to be 0 like with other Lod0 functions, and then the
bias is added to it.

BUG=angle:604

Change-Id: I2dbc309c497d37e960209f08849f9d2bc9e1fbcc
Reviewed-on: https://chromium-review.googlesource.com/194544Tested-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>