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.