poly_int: folding BIT_FIELD_REFs on vectors

This patch makes the:

  (BIT_FIELD_REF CONSTRUCTOR@0 @1 @2)

folder cope with polynomial numbers of elements.

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

gcc/
	* match.pd: Cope with polynomial numbers of vector elements.

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

From-SVN: r256147