Remove GlobalValue::getAlignment().

This function is deceptive at best: it doesn't return what you'd expect.
If you have an arbitrary GlobalValue and you want to determine the
alignment of that pointer, Value::getPointerAlignment() returns the
correct value.  If you want the actual declared alignment of a function
or variable, GlobalObject::getAlignment() returns that.

This patch switches all the users of GlobalValue::getAlignment to an
appropriate alternative.

Differential Revision: https://reviews.llvm.org/D80368