IceCfg::getAssembler() is a template that simply static_casts the CFG's assembler. This could potentially be problematic in the future, so we enabled the (relatively) cheap llvm dyn_cast operator for Assemblers.

This CL also renames assembler_mips32.h to IceAssemblerMIPS32.h.

BUG=
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1211863004.