Test that volatile load type isn't changed

Summary: As discussed in D75505, it's not particularly useful to change the type of a load to/from floating-point/integer because it's followed by a bitcast, and it might lead to surprising code generation. Check that this doesn't generally happen.

Reviewers: lebedev.ri

Subscribers: jkorous, dexonsmith, ributzka, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75644