PR 29585: C++ front end not setting TREE_ADDRESSABLE on vtbl entries

[Diego Novillo]

As discussed in http://gcc.gnu.org/ml/gcc/2007-03/msg01153.html, the C++
front end is building ADDR_EXPRs for vtbl entries but is not setting
TREE_ADDRESSABLE consistently.

This is causing the setup routines in alias analysis to ignore these
variables.  But during analysis, we add them to points-to and alias
sets.  Since the variables had been initially ignored, they don't get
marked for renaming, causing an ICE during conversion to SSA.

The approach I tried initially is to make the aliaser very strict in
asserting the addressability of every symbol added to an alias set.
This uncovers various inconsistencies in C, C++ and Java FEs.  So, the
idea is to make everything consistent by forcing build1() to set
TREE_ADDRESSABLE every time it's asked to build ADDR_EXPR.  This caused
some controversy, so I will not attempt that on 4.2.

The minimal fix that I found for 4.2 is along the lines of what Mark and
Jason suggested.  I did have to try a variation of Mark's suggestion of
building a NOP_EXPR in dfs_accumulate_vtbl_inits, because that was
causing ICEs elsewhere in other functions not expecting a NOP_EXPR.

Bootstrapped on i686, x86_64, ppc64 and ia64.

OK for 4.2?  For 4.3 I will use the sane approach of forcing
TREE_ADDRESSABLE for every ADDR_EXPR built.
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