Optimize common subexpressions in Montgomery multiplication
Optimize common subexpressions in Montgomery multiplication

The hottest loop in the library.
Consistently shaves 5s off a test suite run.

Again, thanks to @daira.

nonconforme/otr,arlolra/otr,sualko/otr,sualko/otr,arlolra/otr,nonconforme/otr,mvayngrib/otr,astorije/otr,mvayngrib/otr,arlolra/otr,astorije/otr,astorije/otr,nonconforme/otr,nonconforme/otr,astorije/otr,sualko/otr,mvayngrib/otr,arlolra/otr,sualko/otr,mvayngrib/otr