[llvm] [MergeFuncs] Use sizeWithoutDebug to decide if we create a thunk (PR #68627)

[Oskar Wirga via llvm-commits]

oskarwirga wrote:

I modified the check from:
```
if (F->front().sizeWithoutDebug() <= 2) {
```
to:
```
if (F->front().sizeWithoutDebug() < 2) {
```
as internal testing has shown that it reduces binary size. 

https://github.com/llvm/llvm-project/pull/68627