spirv/nir: Handle 16-bit types

spirv/nir: Handle 16-bit types

v2: Added more missing implementations of 16-bit types. (Jason Ekstrand)

v3: Store values in values[0].u16[i] (Jason Ekstrand)
    Include switches based on bitsize for 16-bit types
    (Chema Casanova)
v4: Coding style fixes (Jason Ekstrand)
    Use vtn_u64_literal and u64[0] at 64-bit SpvOpConstant (Jason Ekstrand)

Signed-off-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Signed-off-by: Eduardo Lima <elima@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>