Correct reciprocal approximation for power-of-two values.

Correct reciprocal approximation for power-of-two values.

Intel's reciprocal approximation instruction is not exact for power-of-two
values. It provides 12 bits of mantissa precision and keeps a balance between
positive and negative errors, but the reciprocal of 2^x is not 2^-x. This
affects conformance tests which expect varyings not to be affected by the
perspective division. Correct for this by multiplying by the inverse.

Bug 27165393

Change-Id: Ie52ec511a14a4f447adc47ce9c875bbad03cd274
Reviewed-on: https://swiftshader-review.googlesource.com/4903
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>