Only strip symbols if emitting bytecode to the assembly file.

Move lowerselect pass to come after preselection.  Move machine
code construction and stack slots pass to come right before instruction
selection. This is to help fix perlbmk.

Update comments.

Make the sequence of passes in addPassesToJITCompile look more like
the sequence of passes in addPassesToEmitAssembly, including support
for -print-machineinstrs.

llvm-svn: 12614