Revert: r205182 - llvm/test/MC/Mips/mips64r2/valid-xfail.s: This REQUIRES...

Revert: r205182 - llvm/test/MC/Mips/mips64r2/valid-xfail.s: This REQUIRES asserts. Seems it doesn't fail with -Asserts.

This was most likely caused by an uninitialized value and the relevant code was re-written in r205292. Reverting to see if it still fails on any of the buildbots.

llvm-svn: 206033