writebacks to the address register.  This gets rid of the hack that the
first register on the list was the magic writeback register operand.  There
was an implicit constraint that if that operand was not reg0 it had to match
the base register operand.  The post-RA scheduler's antidependency breaker
did not understand that constraint and sometimes changed one without the
other.  This also fixes Radar 7495976 and should help the verifier work
better for ARM code.

There are now new ld/st instructions explicit writeback operands and explicit
constraints that tie those registers together.

llvm-svn: 98409