exp2(x) should be exact for integer values of x. For example exp(1.0)
should produce 2.0.

Change-Id: I9d6d1d388c23e813e09b7b5788c929ff07189357
Reviewed-on: https://swiftshader-review.googlesource.com/16188Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>