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>

src/intel/compiler/brw_compiler.c

@@ -171,6 +171,13 @@ brw_compiler_create(void *mem_ctx, const struct gen_device_info *devinfo)
       fp64_options |= nir_lower_fp64_full_software;
    }
 
+   /* The Bspec's section tittled "Instruction_multiply[DevBDW+]" claims that
+    * destination type can be Quadword and source type Doubleword for Gen8 and
+    * Gen9. So, lower 64 bit multiply instruction on rest of the platforms.
+    */
+   if (devinfo->gen < 8 || devinfo->gen > 9)
+      int64_options |= nir_lower_imul_2x32_64;
+
    /* We want the GLSL compiler to emit code that uses condition codes */
    for (int i = 0; i < MESA_SHADER_STAGES; i++) {
       compiler->glsl_compiler_options[i].MaxUnrollIterations = 0;