intel/fs: add 64 bit integer multiplication lowering

intel/fs: add 64 bit integer multiplication lowering

While NIR's lower_imul64() solves the case of 64 bit integer multiplications
generated early, we don't have a way to lower such instructions when they are
generated by our own backend, such as the scan/reduce intrinsics. We'll need
this soon, so implement it now.

An easy way to test this is to simply disable nir_lower_imul64 to let
those operations reach the backend.

v2:
  - Fix Q/UQ copy/paste errors (Caio).
  - Transform an 'if' into 'else if' (Caio).
  - Add an extra comment to clarify the need for 64b = 32b * 32b
    (Caio).
  - Make private functions private (Caio).
v3:
  - Remove ambiguity with 'b' and 'd' variables (Caio).
  - Allocate potentially less regs for the dwords (Caio).

Cc: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: Matt Turner <matt.turner@intel.com>
Cc: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>