anv: Don't re-use entry_point pointer from spirv_to_nir

anv: Don't re-use entry_point pointer from spirv_to_nir

When running with NIR_TEST_CLONE=1, the pointer will not be valid, as
the whole shader is going to be recreated every pass.  Prefer using
is_entrypoint (to query when looping) and nir_shader_get_entrypoint()
instead.

Fixes the Vulkan Piglit tests
- vulkan/glsl450/frexp-double
- vulkan/glsl450/isinf-double
- vulkan/shaders/fs-multiple-large-local-array

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108957
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>