c: Fix ICE with cast to VLA [93576]

c: Fix ICE with cast to VLA [93576]

The following testcase ICEs, because the PR84305 changes try to evaluate
the size earlier.  If size has side-effects, that is desirable, and the
side-effects will actually be wrapped in a SAVE_EXPR.  The problem on this
testcase is that there are no side-effects, and c_fully_fold doesn't fold
those COMPOUND_EXPRs to constant, and while before gimplification we unshare
trees found in the expressions, the unsharing doesn't involve TYPE_SIZE etc.
of used types.  Gimplification is destructive though, so when we gimplify
the two nested COMPOUND_EXPRs and then try to gimplify it the second time
for the TYPE_SIZEs, we ICE.
Now, we could use unshare_expr in what we push to *expr, SAVE_EXPRs and
their operands in there aren't unshared, but I really don't see a point of
evaluating expressions that don't have side-effects before, so instead
this just pushes there expressions that do have side-effects.

2020-02-13  Jakub Jelinek  <jakub@redhat.com>

	PR c/93576
	* c-decl.c (grokdeclarator): If this_size_varies, only push size into
	*expr if it has side effects.

	* gcc.dg/pr93576.c: New test.