[debug] Reuse debug exprs generated in remap_ssa_name
When compiling gcc.dg/vla-1.c with -O3 -g, vla a and b in f1 are optimized
away, and f1 is cloned to a version f1.constprop with no parameters, eliminating
parameter i. Debug info is generated to describe the sizes of a and b, but
that process generates debug expressions that are not reused.
Fix the duplication by saving and reusing the generated debug expressions in
remap_ssa_name. Concretely: reuse D#7 here instead of generating D#8:
...
__attribute__((noinline))
f1.constprop ()
{
intD.6 iD.1935;