i965/fs: Add brw_reg_type_from_bit_size utility method

i965/fs: Add brw_reg_type_from_bit_size utility method

Returns the brw_type for a given ssa.bit_size, and a reference type.
So if bit_size is 64, and the reference type is BRW_REGISTER_TYPE_F,
it returns BRW_REGISTER_TYPE_DF. The same applies if bit_size is 32
and reference type is BRW_REGISTER_TYPE_HF it returns BRW_REGISTER_TYPE_F

v2 (Jason Ekstrand):
 - Use better unreachable() messages
 - Add Q types

Signed-off-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>