Directly operate on CONST_VECTOR encoding

Directly operate on CONST_VECTOR encoding

This patch makes some pieces of code operate directly on the new
CONST_VECTOR encoding.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* cse.c (hash_rtx_cb): Hash only the encoded elements.
	* cselib.c (cselib_hash_rtx): Likewise.
	* expmed.c (make_tree): Build VECTOR_CSTs directly from the
	CONST_VECTOR encoding.

From-SVN: r256192