Simplify from my last change. Assert1 is a macro that makes its caller return,

Simplify from my last change. Assert1 is a macro that makes its caller return,
so "Assert1(isa<>); cast<>" is a valid idiom.

Actually check the PHI node's odd-numbered operands for BasicBlock-ness, like
the comment said.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81182 91177308-0d34-0410-b5e6-96231b3b80d8