poly_int: expand_expr_real_1

This patch makes the VIEW_CONVERT_EXPR handling in expand_expr_real_1
cope with polynomial type and 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 (expand_expr_real_1): Use tree_to_poly_uint64
	instead of int_size_in_bytes when handling VIEW_CONVERT_EXPRs
	via stack temporaries.  Treat the mode size as polynomial too.

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

From-SVN: r256152