[ConstantHoisting] use struct rather than tuple for adjustments

We pack this info in a tuple just to spread it back out for a function
call. Spreads in C++ are awkward.  If I want to add an additional
element to the tuple, I need to add more calls to std::get<> later. Just
use a struct.

Reviewed By: void

Differential Revision: https://reviews.llvm.org/D155236