gimplifier: handle POLY_INT_CST-sized TARGET_EXPRs

If a TARGET_EXPR has poly-int size, the gimplifier would treat it
like a VLA and use gimplify_vla_decl.  gimplify_vla_decl in turn
would use an alloca and expect all references to be gimplified
via the DECL_VALUE_EXPR.  This caused confusion later in
gimplify_var_or_parm_decl_1 when we (correctly) had direct rather
than indirect references.

For completeness, the patch also fixes similar tests in the RETURN_EXPR
handling and OpenMP depend clauses.

2020-01-17  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* gimplify.c (gimplify_return_expr): Use poly_int_tree_p rather
	than testing directly for INTEGER_CST.
	(gimplify_target_expr, gimplify_omp_depend): Likewise.

gcc/testsuite/
	* g++.target/aarch64/sve/acle/general-c++/gimplify_1.C: New test.