[CVP] Don't assume Constants of type i1 can be known to be true or false

[CVP] Don't assume Constants of type i1 can be known to be true or false

CVP wants to analyze the condition operand of a select along an edge.
It succeeds in getting back a Constant but not a ConstantInt.  Instead,
it gets a ConstantExpr.  It then assumes that the Constant must be equal
to false because it isn't equal to true.

Instead, perform an additional comparison.

This fixes PR23752.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239217 91177308-0d34-0410-b5e6-96231b3b80d8