[debug] Reuse debug exprs generated in remap_ssa_name

[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;

   <bb 3>
   # DEBUG D#10 s=> iD.1897
   # DEBUG iD.1935 => D#10

   <bb 2>
-  # DEBUG D#8 s=> iD.1935
   # DEBUG D#7 s=> iD.1935
   saved_stack.2_1 = __builtin_stack_save ();
   # DEBUG BEGIN_STMT
   # DEBUG D#6 => D#7 + 1
   # DEBUG D#5 => (long intD.8) D#6
   # DEBUG D#4 => D#5 + -1
   # DEBUG D.1937 => (sizetype) D#4
   # DEBUG a.0D.1942 => NULL
   # DEBUG BEGIN_STMT
-  # DEBUG D#3 => D#8 + 2
+  # DEBUG D#3 => D#7 + 2
   # DEBUG D#2 => (long intD.8) D#3
   # DEBUG D#1 => D#2 + -1
   # DEBUG D.1944 => (sizetype) D#1
   # DEBUG b.1D.1949 => NULL
...

Bootstrapped and reg-tested on x86_64.

2018-07-13  Tom de Vries  <tdevries@suse.de>

	* tree-inline.c (remap_ssa_name): Save and reuse debug exprs generated
	in remap_ssa_name.

	* gcc.dg/vla-1.c: Update.

From-SVN: r262628