poly_int: expand_expr_real_2

This patch makes expand_expr_real_2 cope with polynomial mode sizes
when handling conversions involving a union type.

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_2): When handling conversions involving
	unions, apply tree_to_poly_uint64 to the TYPE_SIZE rather than
	multiplying int_size_in_bytes by BITS_PER_UNIT.  Treat GET_MODE_BISIZE
	as a poly_uint64 too.

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

From-SVN: r256151