Check type for legality before forming a select from loads.

Check type for legality before forming a select from loads.

Sorry for the lack of a test case.  I tried writing one for i386 as i know selects are illegal on this target, but they are actually considered legal by isel and expanded later.

I can't see any targets to trigger this, but checking for the legality of a node before forming it is general goodness.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174934 91177308-0d34-0410-b5e6-96231b3b80d8