[X86] Erase some obsolete comments from README.txt

I just tried reproducing some of the optimization failures in README.txt in the
X86 backend, and many of them could not be reproduced. In general the entire
file appears quite bit-rotted, whatever interesting parts remain should be
moved to bugzilla, and the rest deleted. I did not spend the time to do that,
so I just deleted the few I tried reproducing which are obsolete, to save some
time to whoever will find the courage to do it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218170 91177308-0d34-0410-b5e6-96231b3b80d8