nvc0/ir: fix overwriting of value backing non-constant gather offset

nvc0/ir: fix overwriting of value backing non-constant gather offset

Normally the value is an immediate, which is moved to some temporary, so
there's no problem. In the case of a non-constant offset (as allowed by
ARB_gpu_shader5), we have to take care to copy it first before using it
to build up the bits.

This fixes a compilation error observed in F1 2015.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Cc: mesa-stable@lists.freedesktop.org