[sil-llvm-gen] Add support for emitting IR before LLVM optimizations run.
Specifically, I renamed the old llvm-as which printed with optimization as
llvm-as-opt and made llvm-as just test IRGen. That is really the true reason
that I created this utility many years ago: to test how IRGen lowers specific
SIL instruction sequences. How LLVM optimizes such sequences once lowered is a
different problem. I updated the one test that used sil-llvm-gen and required
optimizations to use llvm-as-opt so that the tests output did not change.
(cherry picked from commit 5c8a95f27a75c93395763909733518401e6e223a)