spirv/i965/anv: Relax push constant offset assertions being 32-bit aligned

spirv/i965/anv: Relax push constant offset assertions being 32-bit aligned

The introduction of 16-bit types with VK_KHR_16bit_storages implies that
push constant offsets could be multiple of 2-bytes. Some assertions are
updated so offsets should be just multiple of size of the base type but
in some cases we can not assume it as doubles aren't aligned to 8 bytes
in some cases.

For 16-bit types, the push constant offset takes into account the
internal offset in the 32-bit uniform bucket adding 2-bytes when we access
not 32-bit aligned elements. In all 32-bit aligned cases it just becomes 0.

v2: Assert offsets to be aligned to the dest type size. (Jason Ekstrand)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>