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>
(cherry picked from commit 30b5c9ed)