swift234

[Global Opt] update SILGlobalOpt to use static initializer if possible.

This is off by default and is a modified version of r21996 (reverted in r22001).

Update SILGlobalOpt to remove "once" call from addressor and set the
InitializerF for SILGlboalVariable if legal.

For calls to addressor in non-cold block, we check the addressor to find out
the globalinit_func and the associated SILGlobalVariable. If legal, we set
InitializerF of SILGlobalVariable and remove "once" call from addressor.

The current rules for legality:
1> the addressor contains a single "once" call and it calls globalinit_func.
2> the globalinit_func is called by "once" from a single location.
   When we inline the addressor, the globalinit_func will be called by "once"
   from multiple locations. After we remove the "once" from the addressor, the
   inlined version will mistakely initialize the global variable again.
3> the globalinit_func is trivial as defined by
   SILGlobalVariable::canBeStaticInitializer().

Update IRGen to generate constant initializers for global variables with static
initializers (i.e non-null InitializerF).

Joe's suggestion on having the static initializer to return the result instead
of storing it is not implemented in this commit. But we do verify that a static
initializer has a single store to the global variable. The suggestion is tracked
in rdar://18382039.

Implement rdar://16621368, rdar://18169093.


Swift SVN r22083