[IR] Simplify Use::swap. NFCI.

The new implementation makes it clear that there are exactly two
conditional stores (after the initial no-op optimization). By contrast
the old implementation had seven conditionals, some hidden inside other
functions.

This commit can change the order of operands in operand lists, hence the
tweak to one test case.

Differential Revision: https://reviews.llvm.org/D80116