[CorrelatedValuePropagation] Remove redundant if statement in processSelect()

This statement

    if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType());

is introduced in https://reviews.llvm.org/rG35609d97ae89b8e13f40f4e6b9b056954f8baa83
to fix a case where unreachable code can cause select instruction
simplification to fail. In https://reviews.llvm.org/rGd10480657527ffb44ea213460fb3676a6b1300aa,
we begin to perform a depth-first walk of basic blocks. This means
we will not visit unreachable blocks. So we do not need this the
special check any more.

Differential Revision: https://reviews.llvm.org/D76753