[CodeGen] isInTailCallPosition didn't consider readnone tailcalls

A readnone tailcall may still have a chain of computation which follows
it that would invalidate a tailcall lowering.  Don't skip the analysis
in such cases.

This fixes PR24613.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246304 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed