Use a more accurate implementation for exp

Using exp2(x * M_LOG2E_F) does not give us accurate enough results for
OpenCL.  If you look at the new exp implementation you'll see that
it does multiply the input by M_LOG2E_F, but it still uses the original
input in part of the calculation.

This exp implementation was ported from the AMD builtin library
and has been tested with piglit, OpenCV, and the ocl conformance tests.

llvm-svn: 237229