Add tests for a corner case of when to relax all instructions:

We don't want to relax all instructions in

$ clang -c test.s

since most users don't pass -O when using the driver to assemble.

On the other hand, -save-temps should not change the output unnecessary, so in

$ clang -c test.c -save-temps

we should relax all instructions.

llvm-svn: 169815