Reland test/MC/Mips/reloc-directive-label-offset.s

The test was reverted because it failed on
llvm-clang-x86_64-expensive-checks-win builder, and that was because
-DEXPENSIVE_CHECKS adds randomness to llvm::sort(), affecting the order of
relocation table entries.
Modified the test to not have two relocations at the same offset.

llvm-svn: 347476