[NFC][PhaseOrdering] Add end-to-end tests for the 'two shifts by sext' problem

We start with two separate sext's, but EarlyCSE runs before InstCombine,
so when we get them, they are a single sext, and we just ignore that.
Likewise, if we had a single sext, we don't do anything there.

llvm-svn: 373115