[InferAddressSpaces] NFC: For noop IntToPtr/PtrToInt pair cast to operator instead of PtrToInt

Compiler crashes at an assertion while casting operands to PtrToIntInst at some cases when
ptrtoint is present as an explicit operand to inttoptr. Explicit instruction operator as
operand can not be casted to an Instruction.

This patch replaces cast from PtrToInst to Operator which are later checked for constant
expressions.

Differential Revision: https://reviews.llvm.org/D105002