The SVE support for the new CONST_VECTOR encoding needs to be able
to extract the first N bits of the vector and duplicate it.  This patch
adds a simplify_subreg rule for that.

The code is covered by the gcc.target/aarch64/sve_slp_*.c tests.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* simplify-rtx.c (simplify_immed_subreg): Add an inner_bytes
	parameter and use it instead of GET_MODE_SIZE (innermode).  Use
	inner_bytes * BITS_PER_UNIT instead of GET_MODE_BITSIZE (innermode).
	Use CEIL (inner_bytes, GET_MODE_UNIT_SIZE (innermode)) instead of
	GET_MODE_NUNITS (innermode).  Also add a first_elem parameter.
	Change innermode from fixed_mode_size to machine_mode.
	(simplify_subreg): Update call accordingly.  Handle a constant-sized
	subreg of a variable-length CONST_VECTOR.

From-SVN: r256610