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

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>