Temporarily Revert:

 "[SLP] Generalization of stores vectorization."
 "[SLP] Fix -Wunused-variable. NFC"
 "[SLP] Vectorize jumbled stores."

As they're causing significant (10-30x) compile time regressions on
vectorizable code.

The primary cause of the compile-time regression is f228b537.

This reverts commits:

f228b537
5503455c
21d498c9