[SCEV] SCEVExpander::isHighCostExpansionHelper(): cost-model UDiv by power-of-two as LShr

Summary:
Like with casts, we need to subtract the cost of `lshr` instruction
from budget, and recurse into LHS operand.
Seems "pretty obviously correct" to me?

To be noted, there is a number of other shortcuts we //could// cost-model:
* `... + (-1 * ...)` -> `... - ...` <-  likely very frequent case
* `x - (rem x, power-of-2)`, which is currently `(x udiv power-of-2) * power-of-2` -> `x & -log2(power-of-2)`
* `rem x, power-of-2`, which is currently `x - ((x udiv power-of-2) * power-of-2)` -> `x & log2(power-of-2)-1`
* `... * power-of-2` -> `... << log2(power-of-2)` <- likely not very beneficial

Reviewers: reames, mkazantsev, wmi, sanjoy

Reviewed By: mkazantsev

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73718