nir/glsl: Add another way of doing lower_imul64 for gen8+

nir/glsl: Add another way of doing lower_imul64 for gen8+

On Gen 8 and 9, "mul" instruction supports 64 bit destination type. We
can reduce our 64x64 int multiplication from 4 instructions to 3.

Also instead of emitting two mul instructions, we can emit single mul
instuction and extract low/high 32 bits from 64 bit result for
[i/u]mulExtended

v2: 1) Allow lower_mul_high64 to use new opcode (Jason Ekstrand)
    2) Add lower_mul_2x32_64 flag (Matt Turner)
    3) Remove associative property as bit size is different (Connor
       Abbott)

v3: Fix indentation and variable naming convention (Jason Ekstrand)

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>