Revert "[NFC] factor out unrolling decision logic"

This patch added a requirement for C++17, while LLVM is supposed to
build with C++14
(https://llvm.org/docs/CodingStandards.html#c-standard-versions). Posted
a note to the original review thread (https://reviews.llvm.org/D106001).

This reverts commit 4d559837.

Differential Revision: https://reviews.llvm.org/D108314