[DebugInfo][dexter] Add dexter tests for merged values

These dexter tests illustrate PR48719, the summary of which is:

Sometimes we insert dbg.values for merged values (PHIs) when promoting
variables, sometimes we don't. Sometimes there is no PHI because the merged
value is never used. It doesn't matter because LiveDebugValues understands these
merged values (implicit or otherwise) and correctly updates the debug
info. Importantly, these merged variable values (which may or may not exist as
PHIs, and may or not be represented with dbg.values) are //always// implicitly
defined by the combination of incoming edges and the incoming variable locations
along those edges by virtue of LiveDebugValues existing. Unfortunately, it is
possible to mess with the CFG and remove / move these edges before
LiveDebugValues runs. In this case our debug info model only works when the
merged value is tracked by a dbg.value. Currently, this is only done rigorously
for variables which are A) promoted in the first round of mem2reg and B) are
used after the merge point.

As an example, compile the following source with -O3 -g and step through with a
debugger. You will see parama=5 throughout the function fun which is incorrect -
we expect to see param=20 after the conditional assignment.

    __attribute__((optnone))
    void esc(int* p) {}

    __attribute__((optnone))
    void fluff() {}

    __attribute__((noinline))
    int fun(int parama, int paramb) {
      if (parama)
        parama = paramb;
      fluff();           // DexLabel('s0')
      esc(&parama);
      return 0;
    }

    int main() {
      return fun(5, 20);
    }

1. parama is escaped by esc(&parama) so it is not promoted by
   SROA/mem2reg (failing condition "A" above).
2. InstCombine's LowerDbgDeclare converts the dbg.declare to a set of
   dbg.values (tracking the stored SSA values).
3. InstCombine replaces the two stores to parama's alloca (the initial
   parameter register store in entry and the assignment in if.then) with a
   PHI+store in the common sucessor.
4. SimplifyCFG folds the blocks together and converts the PHI to a
   select.

The debug info is not updated to account for the merged value in the successor
prior to SimplifyCFG when it exists as a PHI, or during when it becomes a
select.

As with D89543, which added some dexter tests for escaped locals, the idea is
to build a set of source-level tests which highlights existing issues and
might be useful in evaluating a new debug info model.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D94761