Remove the LLVM stack promotion pass and related SIL optimization logic.

It's not needed anymore because array buffers are now allocated with alloc_ref instead of a swift_bufferAllocate runtime call.

tools/swift-llvm-opt/LLVMOpt.cpp

@@ -234,7 +234,6 @@ int main(int argc, char **argv) {
   initializeSwiftRCIdentityPass(Registry);
   initializeSwiftARCOptPass(Registry);
   initializeSwiftARCContractPass(Registry);
-  initializeSwiftStackPromotionPass(Registry);
   initializeInlineTreePrinterPass(Registry);
   initializeSwiftMergeFunctionsPass(Registry);