[TTI] `BasicTTIImplBase::getInterleavedMemoryOpCost()`: fix load discounting

The math here is:
Cost of 1 load = cost of n loads / n
Cost of live loads = num live loads * Cost of 1 load
Cost of live loads = num live loads * (cost of n loads / n)
Cost of live loads = cost of n loads * (num live loads / n)

But, all the variables here are integers,
and integer division rounds down,
but this calculation clearly expects float semantics.

Instead multiply upfront, and then perform round-up-division.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112302