Minor tweaks to code computing modular multiplicative inverse

Minor tweaks to code computing modular multiplicative inverse

This removes the last parameter of choose_multiplier, which is unused, adds
another assertion and more details to the description and various comments.
Likewise to the closely related invert_mod2n, except for the last parameter.

[changelog]
	* expmed.h (choose_multiplier): Tweak description and remove last
	parameter.
	* expmed.cc (choose_multiplier): Likewise.  Add assertion for the
	third parameter and adds details to various comments.
	(invert_mod2n): Tweak description and add assertion for the first
	parameter.
	(expand_divmod): Adjust calls to choose_multiplier.
	* tree-vect-generic.cc (expand_vector_divmod): Likewise.
	* tree-vect-patterns.cc (vect_recog_divmod_pattern): Likewise.