nv50/ir: disable tryCollapseChainedMULs in ConstantFolding for precise instructions

fixes dEQP-GLES2.functional.shaders.invariance.mediump.loop_3

CC: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp

       break;
    }
    case OP_MUL:
-      if (i->dType == TYPE_F32)
+      if (i->dType == TYPE_F32 && !i->precise)
          tryCollapseChainedMULs(i, s, imm0);
 
       if (i->subOp == NV50_IR_SUBOP_MUL_HIGH) {