store_bit_field already takes a poly_uint64 size, so we can relax the
INTVAL to rtx_to_poly_int64.  This is tested by the SVE ACLE patches.

2019-10-11  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* expr.c (store_expr): Use rtx_to_poly_int64 rather than
	INTVAL when calling store_bit_field.

From-SVN: r276888