[InstCombine] Fix a crash in getSelectCondition if we happen to have two inverse...

[InstCombine] Fix a crash in getSelectCondition if we happen to have two inverse vectors of i1 constants.

We used to try to truncate the constant vector to vXi1, but if it's already i1 this would fail. Instead we now use IRBuilder::getZExtOrTrunc which should check the type and only create a trunc if needed. I believe this should trigger constant folding in the IRBuilder and ultimately do the same thing just with the additional type check.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310639 91177308-0d34-0410-b5e6-96231b3b80d8