[x86] move code for converting int logic to FP logic to a helper function; NFCI

[x86] move code for converting int logic to FP logic to a helper function; NFCI

This is a follow-on to:
http://reviews.llvm.org/rL248395

so we can add the call to the or/xor combines too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248399 91177308-0d34-0410-b5e6-96231b3b80d8