nir/opt_copy_prop_vars: Quiet valgrind warning about overlapping memcpy.

nir/opt_copy_prop_vars: Quiet valgrind warning about overlapping memcpy.

The warning is kind of silly:

Test case 'dEQP-GLES2.functional.shaders.indexing.tmp_array.vec3_const_write_static_read_vertex'..
==1874780== Source and destination overlap in memcpy(0xa261690, 0xa261690, 160)
==1874780==    at 0x484D498: __GI_memcpy (vg_replace_strmem.c:1037)
==1874780==    by 0x596FC07: copy_entry_remove (nir_opt_copy_prop_vars.c:296)

The "memcpy is undefined if they overlap" thing is surely meant to be
"memcpy with *partial* overlap is undefined", but let's keep anyone else
from having to debug this.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6178>