[RS4GC] Turn lambda into static function. NFC.

Extract `rematerializeChain()` lambda into static function.
We'll need it in upcoming patch to RS4GC pass.
There is small interface change: now reversal of `ChainToBase` is
performed within this function, not outside.
Still this is non-functional change.

Reviewed By: skatkov

Differential Revision: https://reviews.llvm.org/D138910