Revert part of r141274. Only need to change encoding for xchg %eax, %eax in 64-bit...

Revert part of r141274. Only need to change encoding for xchg %eax, %eax in 64-bit mode. This is because in 64-bit mode xchg %eax, %eax implies zeroing the upper 32-bits of RAX which makes it not a NOP. In 32-bit mode using NOP encoding is fine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141353 91177308-0d34-0410-b5e6-96231b3b80d8