[X86] Split out the logic to select the stack probe function (NFC)

Patch 1 of 11 in 'Add a "probe-stack" attribute' review thread.

Patch by: <john.kare.alsaker@gmail.com>



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216233 91177308-0d34-0410-b5e6-96231b3b80d8