[DebugInfo] Replace debug uses in replaceUsesOutsideBlock

Value::replaceUsesOutsideBlock doesn't replace debug uses which leads to an
unnecessary reduction in variable location coverage. Fix this, add a unittest for
it, and add a regression test demonstrating the change through instcombine's
replacedSelectWithOperand.

Reviewed By: djtodoro

Differential Revision: https://reviews.llvm.org/D99169