intel/compiler: grf127 can not be dest when src and dest overlap in send

Implement at brw_eu_validate the restriction from Intel Broadwell PRM,
vol 07, section "Instruction Set Reference", subsection "EUISA
Instructions", Send Message (page 990):

"r127 must not be used for return address when there is a src and
dest overlap in send instruction."

v2: Style fixes (Matt Turner)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Cc: 18.1 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 0e47ecb2)