brw: brw_reg::nr for an accumulator is not part of the offset

Without this, reg_offset will return 1024 for acc0. This causes
has_invalid_dst_region to decide that the destination region is invalid
(because 1024 != 0), and the lowering code tries to treat the floating
point accumulators as integers. It's a mess.

v2: Add and use set_gfx_platform. Suggested by Caio.

Fixes: 937373eb25 ("i965/fs: Handle fixed HW GRF subnr in reg_offset().")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
(cherry picked from commit cfdb3ddb93)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40979>

.pick_status.json

@@ -3344,7 +3344,7 @@
         "description": "brw: brw_reg::nr for an accumulator is not part of the offset",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "937373eb25c53b0803e526b7b273afc2b3330752",
         "notes": null

src/intel/compiler/brw/brw_reg.h

@@ -1447,7 +1447,8 @@ reg_space(const brw_reg &r)
 static inline unsigned
 reg_offset(const brw_reg &r)
 {
-   return (r.file == ADDRESS || r.file == VGRF || r.file == IMM || r.file == ATTR ? 0 : r.nr) *
+   return (r.file == ADDRESS || r.file == VGRF || r.file == IMM ||
+           r.file == ATTR || brw_reg_is_arf(r, BRW_ARF_ACCUMULATOR) ? 0 : r.nr) *
           (r.file == UNIFORM ? 4 : REG_SIZE) + r.offset +
           (r.file == ADDRESS || r.file == ARF || r.file == FIXED_GRF ? r.subnr : 0);
 }

src/intel/compiler/brw/test_helpers.h

@@ -128,6 +128,18 @@ protected:
       brw_init_isa_info(&compiler->isa, devinfo);
    }
 
+   void
+   set_gfx_platform(const char *name)
+   {
+      int pci_id = intel_device_name_to_pci_device_id(name);
+      assert(pci_id > 0);
+
+      intel_get_device_info_from_pci_id(pci_id, devinfo);
+      assert(devinfo->ver > 0);
+
+      brw_init_isa_info(&compiler->isa, devinfo);
+   }
+
    brw_builder
    make_shader(mesa_shader_stage stage = MESA_SHADER_FRAGMENT,
                unsigned dispatch_width = 0)