[CostModel] Express cost(urem) as cost(div+mul+sub) when set to Expand.

The Legalizer expands the operations of urem/srem into a div+mul+sub or divrem
when those are legal/custom. This patch changes the cost-model to reflect that
cost.

Since there is no 'divrem' Instruction in LLVM IR, the cost of divrem
is assumed to be the same as div+mul+sub since the three operations will
need to be executed at runtime regardless.

Patch co-authored by David Sherwood (@david-arm)

Reviewed By: RKSimon, paulwalker-arm

Differential Revision: https://reviews.llvm.org/D103799