[x86] move combines of 'select of 2 constants' to its own function; NFC

There are missing folds here and possibly folds that could be made generic.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280817 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed