nir/spirv: Make vtn_get_builtin_location smarter

nir/spirv: Make vtn_get_builtin_location smarter

Instead of just stomping on the mode, it now validates asserts that the
previously set mode is correct and only changes it if needed.  We need to
do this because, in geometry shaders, there are some builtins that can be
either an input or an output depending on context.  We can get that
information from the SPIR-V source but we can't throw it away.