nv50/ir: r63 is only 0 if we are using less than 63 registers

nv50/ir: r63 is only 0 if we are using less than 63 registers

It is advantageous to use r63 instead of r127 since r63 can fit into the
shorter encoding. However if we've RA'd over 63 registers, we must use
r127 as the replacement instead.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: "11.0" <mesa-stable@lists.freedesktop.org>