swift234

Don't delete debug_value/debug_value_addr in dead code elimination unless their definitions are dead.

And also adapt a whole set of SIL passes so that they can deal with (the not deleted) debug_value instructions.
This was required to prevent perforamnce and code size regressions.
Now the generated code is (almost) the same as before.

The effect of this change is that we keep debug_value/debug_value_addr also in optimized code (more or less).
Fixes rdar://problem/18709125.




Swift SVN r28872