If you don't have HW to do bfi, then lowering bitfieldInsert to bfi makes
things harder than keeping the "bits" argument around.
This still uses bfm, but I've added the obvious lowering of bfm if you
need it.
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>