[CodeGen] Add -align-loops

to `lib/CodeGen/CommandFlags.cpp`. It can replace
-x86-experimental-pref-loop-alignment=.

The loop alignment is only used by MachineBlockPlacement.
The implementation uses a new `llvm::TargetOptions` for now, as
an IR function attribute/module flags metadata may be overkill.

This is the llvm part of D106701.