mesa: Silence 'left shift of negative value' warning in BPTC compression code

mesa: Silence 'left shift of negative value' warning in BPTC compression code

src/util/format/../../mesa/main/texcompress_bptc_tmp.h:830:31: warning: left shift of negative value [-Wshift-negative-value]
  830 |       value |= (~(int32_t) 0) << n_bits;
      |                               ^~

v2: Rewrite to just shift left then shift right.  Based on conversation
with Neil in
https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2792#note_320272,
this should be fine.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com> [v1]
Reviewed-by: Neil Roberts <nroberts@igalia.com>