i965/fs: use the NIR bit size when creating registers

i965/fs: use the NIR bit size when creating registers

v2 (Iago):
  - Squashed bits from 'support double precission constant operands for
    the implementation of 64-bit emit_load_const'.
  - Do not use BRW_REGISTER_TYPE_D for all 32-bit registers since that breaks
    asserts and functionality for some piglit tests. Just keep 32-bit types
    untouched and add 64-bit support.
  - Use DF instead of Q for 64-bit registers. Otherwise the code we generate
    will use Q sometimes and DF others and we hit unwanted DF/Q conversions,
    so always use DF.

v3 (Sam):
  - Mark 'reg_type' occurrences as const (Topi).

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Signed-off-by: Tapani Palli <tapani.palli@intel.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Signed-off-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>