Revert "Revert "[SCEV] Add SCEVType to represent `vscale`.""

Relanding after fixing Polly related build error.

This reverts commit 7b26dcae9eaf8cdcba7fef032fd83d060dffd4b4.

GitOrigin-RevId: 62d11b2ccaeec8abc12e07aa009c922d72fd6377
4 files changed