Previously, in order to get things working, we just always shadowed
variables. Now, we rewrite derefs whenever it's safe to do so and only
shadow if we have an in or out variable that we write or read to
respectively.
Parameterize build_asin() on the fit coefficients so the
implementation can be shared while still using different polynomials
for asin and acos. Also switch back to implementing acos in terms of
asin -- The improvement obtained from cancelling out the pi/2 terms
was negligible compared to the approximation error.
vtn_handle_type() creates a signed type regardless of the value of the
signedness flag, which usually doesn't make much of a difference
except when the type is used as base sampled type of an image type,
what will cause the base type of the NIR image variable to be
inconsistent with its format and cause an assertion failure in the
back-end (most likely only reproducible on Gen7), and may change the
semantics of the image intrinsic subtly (e.g. UMIN may become IMIN).
