This extends the invalid cases we catch in const prop to include
overflowing integer casts using the same machinery as the overflowing
binary and unary operation logic.
LLVM 7 is over a year old, which should be plenty for compatibility. The
last LLVM 6 holdout was llvm-emscripten, which went away in #65501.
I've also included a fix for LLVM 8 lacking `MemorySanitizerOptions`,
which was broken by #66522.
