Reapply "[MBP] Reduce code size by running tail merging in MBP.""

This reapplies commit r271930, r271915, r271923.  They hit a bug in
Thumb which is fixed in r272258 now.

The original message:

The code layout that TailMerging (inside BranchFolding) works on is not the
final layout optimized based on the branch probability. Generally, after
BlockPlacement, many new merging opportunities emerge.

This patch calls Tail Merging after MBP and calls MBP again if Tail Merging
merges anything.

llvm-svn: 272267