Move whole-program virtual call optimization pass after function attribute...

Move whole-program virtual call optimization pass after function attribute inference in LTO pipeline.

As a result of D18634 we no longer infer certain attributes on linkonce_odr
functions at compile time, and may only infer them at LTO time. The readnone
attribute in particular is required for virtual constant propagation (part
of whole-program virtual call optimization) to work correctly.

This change moves the whole-program virtual call optimization pass after
the function attribute inference passes, and enables the attribute inference
passes at opt level 1, so that virtual constant propagation has a chance to
work correctly for linkonce_odr functions.

Differential Revision: http://reviews.llvm.org/D20643

llvm-svn: 270765