Fix bogus CONST_WIDE_INT hash

Fix bogus CONST_WIDE_INT hash

The CONST_WIDE_INT case in const_rtx_hash_1 started the hash
with the precision of the mode, but the mode is always VOIDmode.

2017-08-21  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* varasm.c (const_rtx_hash_1): Don't hash in the mode of a
	CONST_WIDE_INT.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251218