When we use llvm-gcc without using –C, llvm tools and optimizations
are not used at all.

llvm-gcc uses standard LLVM codegenerators. LLVM optimization passes are
enabled via -O1, -O2, -O3 options

By the way, Does llvmc works on the bitcode format?

Yes, it should.

Therefore, all optimizations of LLVM is on bitcode format described in
LLVM Language Reference Manual ?

Consider looking at http://llvm.org/docs/Passes.html