Add more vec_duplicate simplifications

Add more vec_duplicate simplifications

This patch adds a vec_duplicate_p helper that tests for constant
or non-constant vector duplicates.  Together with the existing
const_vec_duplicate_p, this complements the gen_vec_duplicate
and gen_const_vec_duplicate added by a previous patch.

The patch uses the new routines to add more rtx simplifications
involving vector duplicates.  These mirror simplifications that
we already do for CONST_VECTOR broadcasts and are needed for
variable-length SVE, which uses:

  (const:M (vec_duplicate:M X))

to represent constant broadcasts instead.  The simplifications do
trigger on the testsuite for variable duplicates too, and in each
case I saw the change was an improvement.

The best way of testing the new simplifications seemed to be
via selftests.  The patch cribs part of David's patch here:
https://gcc.gnu.org/ml/gcc-patches/2016-07/msg00270.html .

2017-11-01  Richard Sandiford  <richard.sandiford@linaro.org>
	    David Malcolm  <dmalcolm@redhat.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* rtl.h (vec_duplicate_p): New function.
	* selftest-rtl.c (assert_rtx_eq_at): New function.
	* selftest-rtl.h (ASSERT_RTX_EQ): New macro.
	(assert_rtx_eq_at): Declare.
	* selftest.h (selftest::simplify_rtx_c_tests): Declare.
	* selftest-run-tests.c (selftest::run_tests): Call it.
	* simplify-rtx.c: Include selftest.h and selftest-rtl.h.
	(simplify_unary_operation_1): Recursively handle vector duplicates.
	(simplify_binary_operation_1): Likewise.  Handle VEC_SELECTs of
	vector duplicates.
	(simplify_subreg): Handle subregs of vector duplicates.
	(make_test_reg, test_vector_ops_duplicate, test_vector_ops)
	(selftest::simplify_rtx_c_tests): New functions.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Malcolm <dmalcolm@redhat.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r254294