intel/vec4: Don't spill fp64 registers more than once

The way we handle spilling for fp64 in vec4 is to emit a series of MOVs
which swizzles the data around and then a pair of 32-bit spills.  This
works great except that the next time we go to pick a spill reg, the
compiler isn't smart enough to figure out that the register has already
been spilled.  Normally we do this by looking at the sources of spill
instructions (or destinations of fills) but, because it's separated from
the actual value by a MOV, we can't see it.  This commit adds a new
opcode VEC4_OPCODE_MOV_FOR_SCRATCH which is identical to MOV in
semantics except that it lets RA know not to spill again.

Fixes: 82c69426a5 "i965/vec4: support basic spilling of 64-bit registers"
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10571>

src/intel/compiler/brw_ir_performance.cpp

@@ -379,6 +379,7 @@ namespace {
       case BRW_OPCODE_ADD:
       case BRW_OPCODE_MUL:
       case SHADER_OPCODE_MOV_RELOC_IMM:
+      case VEC4_OPCODE_MOV_FOR_SCRATCH:
          if (devinfo->ver >= 11) {
             return calculate_desc(info, unit_fpu, 0, 2, 0, 0, 2,
                                   0, 10, 6, 14, 0, 0);