[CostModel] Use operands argument in getInstructionCost in more places

The current instruction's pointer operand may be different from the one
specified in the Operands argument. We should use the pointer operand
from here instead in case the user has transformed it.

This manifested itself somewhere down the line in
https://reviews.llvm.org/D149889, but I haven't been able to create a
test case on its own yet unfortunately.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D153574