[MergeFunctions] Fix merging of small weak functions

[MergeFunctions] Fix merging of small weak functions

When two interposable functions are merged, we cannot replace
uses and have to emit calls to a common internal function. However,
writeThunk() will not actually emit a thunk if the function is too
small. This leaves us in a broken state where mergeTwoFunctions
already rewired the functions, but writeThunk doesn't do anything.

This patch changes the implementation so that:

 * writeThunk() does just that.
 * The direct replacement of calls is moved into mergeTwoFunctions()
   into the non-interposable case only.
 * isThunkProfitable() is extracted and will be called for
   the non-iterposable case always, and in the interposable case
   only if uses are still left after replacement.

This issue has been introduced in https://reviews.llvm.org/D34806,
where the code for checking thunk profitability has been moved.

Differential Revision: https://reviews.llvm.org/D46804

Reviewed By: whitequark

llvm-svn: 332342