poly_int: emit_single_push_insn_1

This patch makes emit_single_push_insn_1 cope with polynomial mode sizes.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* expr.c (emit_single_push_insn_1): Treat mode sizes as polynomial.
	Use plus_constant instead of gen_rtx_PLUS.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r256154