Revert svn 122743, removing the instcombine pass that was replaced by earlycse.

My i386 llvm-gcc nightly tester found a regression for
SingleSource/Benchmarks/McGill/chomp that a bisect blamed on 122743.
That seems strange but apparently the combination of earlycse and instcombine
did something bad.  Chris says he intended to remove the instcombine pass, so
let's go ahead and try that.  We'll see if there are any performance losses.

llvm-svn: 122907