math: Add tan implementation

Uses the algorithm:
tan(x) = sin(x) / sqrt(1-sin^2(x))

An alternative is:
tan(x) = sin(x) / cos(x)

Which produces more verbose bitcode and longer assembly.

Either way, the generated bitcode seems pretty nasty and a more optimized
but still precise-enough solution is welcome.

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 217511