[AArch64] Fix sve/extract_[12].c for big-endian SVE

[AArch64] Fix sve/extract_[12].c for big-endian SVE

sve/extract_[12].c were relying on the target-independent optimisation
that removes a redundant vec_select, so that we don't end up with
things like:

    dup v0.4s, v0.4s[0]
    ...use s0...

But that optimisation rightly doesn't trigger for big-endian targets,
because GCC expects lane 0 to be in the high part of the register
rather than the low part.

SVE breaks this assumption -- see the comment at the head of
aarch64-sve.md for details -- so the optimisation is valid for
both endiannesses.  Long term, we probably need some kind of target
hook to make GCC aware of this.

But there's another problem with the current extract pattern: it doesn't
tell the register allocator how cheap an extraction of lane 0 is with
tied registers.  It seems better to split the lane 0 case out into
its own pattern and use tied operands for the FPR<-SIMD case,
so that using different registers has the cost of an extra reload.
I think we want this for both endiannesses, regardless of the hook
described above.

Also, the gen_lowpart in this pattern fails for aarch64_be due to
TARGET_CAN_CHANGE_MODE_CLASS restrictions, so the patch uses gen_rtx_REG
instead.  We're only creating this rtl in order to print it, so there's
no need for anything fancier.

2018-01-30  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/aarch64/aarch64-sve.md (*vec_extract<mode><Vel>_0): New
	pattern.
	(*vec_extract<mode><Vel>_v128): Require a nonzero lane number.
	Use gen_rtx_REG rather than gen_lowpart.

Reviewed-by: James Greenhalgh <james.greenhalgh@arm.com>
From-SVN: r257178