[CodeGen][BasicBlockSections] Fix wrong alignment directive placement in

basic block section cases

MachineBlockPlacement pass sets an alignment attribute to the loop
header MBB and this attribute will lead to an alignment directive during
emitting asm. In the case of the basic block section, the alignment
directive is put before the section label, and thus the alignment is set
to the predecessor of the loop header, which is not what we expect and
increases the code size (both inserting nop and set section alignment).

Reviewed By: rahmanl

Differential Revision: https://reviews.llvm.org/D137535