Branch Folding: Accept explicit threshold for tail merge size.

This is prep work for allowing the threshold to be different during layout,
and to enforce a single threshold between merging and duplicating during
layout. No observable change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279117 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed