Simplify ec_GFp_simple_points_make_affine.

Replace the tree-like structure by a linear approach, with fewer special
cases to handle value 0.

(Imported from upstream's d5213519c0ed87c71136084e7e843a4125ecc024.)

Change-Id: Icdd4815066bdbab0d2c0020db6a8cacc49b3d82a
Reviewed-on: https://boringssl-review.googlesource.com/1400
Reviewed-by: Adam Langley <agl@google.com>