nv50/ir: fix lowering of predicated instructions (without defs)

Note that predicated instructions with defs are still not supported
because transformation to SSA doesn't handle them yet.

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "10.2" <mesa-stable@lists.freedesktop.org>

src/gallium/drivers/nouveau/codegen/nv50_ir_lowering_nv50.cpp

@@ -1279,8 +1279,11 @@ NV50LoweringPreSSA::checkPredicate(Instruction *insn)
    Value *pred = insn->getPredicate();
    Value *cdst;
 
-   if (!pred || pred->reg.file == FILE_FLAGS)
+   // FILE_PREDICATE will simply be changed to FLAGS on conversion to SSA
+   if (!pred ||
+       pred->reg.file == FILE_FLAGS || pred->reg.file == FILE_PREDICATE)
       return;
+
    cdst = bld.getSSA(1, FILE_FLAGS);
 
    bld.mkCmp(OP_SET, CC_NEU, insn->dType, cdst, insn->dType, bld.loadImm(NULL, 0), pred);