[debug] Handle debug references to skipped params

[debug] Handle debug references to skipped params

When compiling guality/vla-1.c with -O3 -g, vla a in f1 is optimized away, but
f1 still contains a debug expression describing the upper bound of the vla
(D.1914):
...
     __attribute__((noinline))
     f1 (intD.6 iD.1900)
     {
       <bb 2>
       saved_stack.1_2 = __builtin_stack_save ();
       # DEBUG BEGIN_STMT
       # DEBUG D#3 => i_1(D) + 1
       # DEBUG D#2 => (long intD.8) D#3
       # DEBUG D#1 => D#2 + -1
       # DEBUG D.1914 => (sizetype) D#1
...

Then f1 is cloned to a version f1.constprop with no parameters, eliminating
parameter i, and 'DEBUG D#3 => i_1(D) + 1' turns into 'D#3 => NULL'.

This patch fixes that by defining debug expressions for default defs of
eliminated parameters in remap_ssa_name:
...
     __attribute__((noinline))
     f1.constprop ()
     {
       intD.6 iD.1949;

       <bb 3>
       # DEBUG D#8 s=> iD.1900
       # DEBUG iD.1949 => D#8

       <bb 2>
+      # DEBUG D#6 s=> iD.1949
       saved_stack.1_1 = __builtin_stack_save ();
       # DEBUG BEGIN_STMT
-      # DEBUG D#3 => NULL
+      # DEBUG D#3 => D#6 + 1
       # DEBUG D#2 => (long intD.8) D#3
       # DEBUG D#1 => D#2 + -1
       # DEBUG D.1951 => (sizetype) D#1
...

Bootstrapped and reg-tested on x86_64.

2018-07-09  Tom de Vries  <tdevries@suse.de>

	* cfgexpand.c (expand_debug_source_expr): Handle VAR_DECL.
	* tree-inline.c (remap_ssa_name): Handle default def ssa_name mapping
	onto VAR_DECL with abstract origin.

	* gcc.dg/vla-1.c: New test.

From-SVN: r262510