i965/fs: unspills shoudn't use grf127 as dest since Gen8+

At 232ed89802 "i965/fs: Register allocator
shoudn't use grf127 for sends dest" we didn't take into account the case
of SEND instructions that are not send_from_grf. But since Gen7+ although
the backend still uses MRFs internally for sends they are finally
assigned to a GRFs.

In the case of unspills the backend assigns directly as source its
destination because it is suppose to be available. So we always have a
source-destination overlap. If the reg_allocator assigns registers that
include the grf127 we fail the validation rule that affects Gen8+
"r127 must not be used for return address when there is a src and dest
overlap in send instruction."

So this patch activates the grf127_send_hack_node for Gen8+ and if we
have any register spilled we add interferences to the destination of
the unspill operations.

We also need to avoid that opt_bank_conflicts() optimization, that runs
after the register allocation, doesn't move things around, causing the
grf127 to be used in the condition we were avoiding.

Fixes piglit test tests/spec/arb_compute_shader/linker/bug-93840.shader_test
and some shader-db crashed because of the grf127 validation rule..

v2: make sure that opt_bank_conflicts() optimization doesn't change
the use of grf127. (Caio)

Found by Caio Marcelo de Oliveira Filho

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107193
Fixes: 232ed89802 "i965/fs: Register allocator shoudn't use grf127 for sends dest"
Cc: 18.1 <mesa-stable@lists.freedesktop.org>
Cc: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

src/intel/compiler/brw_fs_bank_conflicts.cpp

@@ -540,6 +540,18 @@ namespace {
       for (unsigned reg = 0; reg < 2; reg++)
          constrained[p.atom_of_reg(reg)] = true;
 
+      /* At Intel Broadwell PRM, vol 07, section "Instruction Set Reference",
+       * subsection "EUISA Instructions", Send Message (page 990):
+       *
+       * "r127 must not be used for return address when there is a src and
+       * dest overlap in send instruction."
+       *
+       * Register allocation ensures that, so don't move 127 around to avoid
+       * breaking that property.
+       */
+      if (v->devinfo->gen >= 8)
+         constrained[p.atom_of_reg(127)] = true;
+
       foreach_block_and_inst(block, fs_inst, inst, v->cfg) {
          /* Assume that anything referenced via fixed GRFs is baked into the
           * hardware's fixed-function logic and may be unsafe to move around.