[NFC] Rename enable-strict-reductions to force-ordered-reductions

I'm renaming the flag because a future patch will add a new
enableOrderedReductions() TTI interface and so the meaning of this
flag will change to be one of forcing the target to enable/disable
them. Also, since other places in LoopVectorize.cpp use the word
'Ordered' instead of 'strict' I changed the flag to match.

Differential Revision: https://reviews.llvm.org/D107264