Recommit "[InstCombine] Improve bswap optimization" (2nd try)

Issue was an assertion failure due to an unchecked `cast`. Fix is to
check the operator is `BinaryOperator` before cast so that we won't
match `ConstExpr`

Reviewed By: goldstein.w.n, RKSimon

Differential Revision: https://reviews.llvm.org/D149699