Added tests for denormal and underflow for simd exp()

The SIMD math functions are not expected to handle
the denormal range correctly, but it should at least
be clamped to zero rather than producing garbage
values. No changes required to the math function,
so it appears to pass these tests as-is.

Change-Id: I841c2948c81f6b62f42e60ff30ffd5e4f77926fa