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
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)