[InstCombine] Fix a vector splat handling bug in transformZExtICmp.

[InstCombine] Fix a vector splat handling bug in transformZExtICmp.

We were using an i1 type and then zero extending to a vector. Instead just create the 0/1 directly as a ConstantInt with the correct type. No need to ask ConstantExpr to zero extend for us.

This bug is a bit tricky to hit because it requires us to visit a zext of an icmp that would normally be simplified to true/false, but that icmp hasnt' been visited yet. In the test case this zext and icmp were created by visiting a udiv and due to worklist ordering we got to the zext first.

Fixes PR34841.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314971 91177308-0d34-0410-b5e6-96231b3b80d8