Refactor BN_mod_mul_montgomery().

Refactor BN_mod_mul_montgomery().

Pull out the simplistic implementation (using BN_mul() or BN_sqr()) into a
bn_mod_mul_montgomery_simple() function. Provide bn_mod_mul_montgomery()
with an implementation that changes depending on if the assembly
bn_mul_mont() is available or not. Turn BN_mod_mul_montgomery() and
BN_to_montgomery() into callers of bn_mod_mul_montgomery().

ok beck@ tb@