Teach SimplifyCFG to fold switches into lookup tables in more cases.

In particular, it couldn't handle cases where lookup table constant
expressions involved bitcasts. This does not seem to come up
frequently in C++, but comes up reasonably often in Rust via
`#[derive(Debug)]`.

Originally reported by pcwalton.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D109565