Hexagon: fix F2_conv_* instructions for negative zero

Hexagon: fix F2_conv_* instructions for negative zero

The implementation for these instructions handles -0 as an invalid float
point value, whereas the Hexagon hardware considers it the same as +0
(which is valid). Let's fix that and add a regression test.

Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Reviewed-by: Brian Cain <bcain@quicinc.com>
Reviewed-by: Taylor Simpson <ltaylorsimpson@gmail.com>
Signed-off-by: Brian Cain <bcain@quicinc.com>