nir: Remove series of unnecessary conversions

nir: Remove series of unnecessary conversions

Clang warns:

warning: absolute value function 'fabsf' given an argument of type
'const float64_t' (aka 'const double') but has parameter of type 'float'
which may cause truncation of value [-Wabsolute-value]

            float64_t dst = bit_size == 64 ? fabs(src0) : fabsf(src0);

The type of the ternary expression will be the common type of fabs() and
fabsf(): double. So fabsf(src0) will be implicitly converted to double.
We may as well just convert src0 to double before a call to fabs() and
remove the needless complexity, à la

            float64_t dst = fabs(src0);

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>