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

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.

llvm-svn: 141353