nir: Delete most of the constant_initializer support

nir: Delete most of the constant_initializer support

Constant initializers have been a constant (ha!) pain for quite some time.
While they're useful from a language perspective, people writing passes or
backends really don't want deal with them most of the time.  This commit
removes most of the constant initializer support from NIR.  It is expected
that you call nir_lower_constant_initializers VERY EARLY to ensure that
they're gone before you do anything interesting.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>