[mips] Revert fixes for PR32020.

The fix introduces segfaults and clobbers the value to be stored when
the atomic sequence loops.

Revert "[Target/MIPS] Kill dead code, no functional change intended."

This reverts commit r296153.

Revert "Recommit "[mips] Fix atomic compare and swap at O0.""

This reverts commit r296134.

llvm-svn: 297380