i965/vec4: Ignore swizzle of VGRF for use by var_range_end()

Issue description from Matt's commit e7c376ad:
 "var_range_end(v, n) loops over the n components of variable number v and
  finds the maximum value, giving the last use of any component of v.
  Therefore it expects v to correspond to the variable associated with the
  .x channel of the VGRF.

  var_from_reg() however returns the variable for the first channel of the
  VGRF, post-swizzle.

  So, if the last register had a swizzle with y, z, or w in the swizzle
  component, we would read out of bounds. For any other register, we would
  read liveness information from the next register.

  The fix is to convert the src_reg to a dst_reg in order to call the
  dst_reg version of var_from_reg() that doesn't consider the swizzle."

Closes: #3003
Fixes: 48dfb30f ('intel/compiler: Move all live interval analysis results into vec4_live_variables')
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Andrii Simiklit <asimiklit.work@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4941>

src/intel/compiler/brw_vec4_cse.cpp

@@ -288,7 +288,7 @@ vec4_visitor::opt_cse_local(bblock_t *block, const vec4_live_variables &live)
              * more -- a sure sign they'll fail operands_match().
              */
             if (src->file == VGRF) {
-               if (live.var_range_end(var_from_reg(alloc, *src), 8) < ip) {
+               if (live.var_range_end(var_from_reg(alloc, dst_reg(*src)), 8) < ip) {
                   entry->remove();
                   ralloc_free(entry);
                   break;