[RTC] Split alias groups according to read only base addresses

  If there are multiple read only base addresses in an alias group
  we can split it into multiple alias groups each with only one
  read only access. This way we might reduce the number of
  comparisons significantly as it grows linear in the number of
  alias groups but exponential in their size.

Differential Revision: http://reviews.llvm.org/D5435

llvm-svn: 218757