Add a small pass that sets the readnone/readonly

Add a small pass that sets the readnone/readonly
attributes on functions, based on the result of
alias analysis.  It's not hardwired to use
GlobalsModRef even though this is the only (AFAIK)
alias analysis that results in this pass actually
doing something.  Enable as follows:
  opt ... -globalsmodref-aa -markmodref ...
Advantages of this pass: (1) records the result
of globalsmodref in the bitcode, meaning it is
available for use by later passes (currently
the pass manager isn't smart enough to magically
make an advanced alias analysis available to all
later passes), which may expose more optimization
opportunities; (2) hopefully speeds up compilation
when code is optimized twice, for example when a
file is compiled to bitcode, then later LTO is done
on it: marking functions readonly/readnone when
producing the initial bitcode should speed up alias
analysis during LTO; (3) good for discovering that
globalsmodref doesn't work very well :)
Not currently turned on by default.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55604 91177308-0d34-0410-b5e6-96231b3b80d8