math: Add asin implementation

asin(x) = atan2(x, sqrt( 1-x^2 ))

alternatively:
asin(x) = PI/2 - acos(x)

Use the atan2 implementation since it produces slightly shorter bitcode and
R600 machine code.

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 217510