[MergeFunctions] Merge small functions if possible without a thunk.

[MergeFunctions] Merge small functions if possible without a thunk.

This can result in significant code size savings in some cases,
e.g. an interrupt table all filled with the same assembly stub
in a certain Cortex-M BSP results in code blowup by a factor of 2.5.

Differential Revision: https://reviews.llvm.org/D34806

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315853 91177308-0d34-0410-b5e6-96231b3b80d8