[AArch64] Remove use of wider vector modes

[AArch64] Remove use of wider vector modes

The AArch64 port defined x2, x3 and x4 vector modes that were only used
in the rtl for the AdvSIMD LD{2,3,4} patterns.  It seems unlikely that
this rtl would have led to any valid simplifications, since the values
involved were unspecs that had a different number of operands from the
non-dreg versions.  (The dreg UNSPEC_LD2 had a single operand, while
the qreg one had two operands.)

As it happened, the patterns led to invalid simplifications on big-
endian targets due to a mix-up in the operand order, see Tamar's fix
in r240271.

This patch therefore replaces the rtl patterns with dedicated unspecs.
This allows the x2, x3 and x4 modes to be removed, avoiding a clash
with 256-bit and 512-bit SVE.

2017-08-22  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* config/aarch64/aarch64-modes.def: Remove 32-, 48- and 64-byte
	vector modes.
	* config/aarch64/iterators.md (VRL2, VRL3, VRL4): Delete.
	* config/aarch64/aarch64.md (UNSPEC_LD2_DREG, UNSPEC_LD3_DREG)
	(UNSPEC_LD4_DREG): New unspecs.
	* config/aarch64/aarch64-simd.md (aarch64_ld2<mode>_dreg_le)
	(aarch64_ld2<mode>_dreg_be): Replace with...
	(aarch64_ld2<mode>_dreg): ...this pattern and use the new DREG
	unspec.
	(aarch64_ld3<mode>_dreg_le)
	(aarch64_ld3<mode>_dreg_be): Replace with...
	(aarch64_ld3<mode>_dreg): ...this pattern and use the new DREG
	unspec.
	(aarch64_ld4<mode>_dreg_le)
	(aarch64_ld4<mode>_dreg_be): Replace with...
	(aarch64_ld4<mode>_dreg): ...this pattern and use the new DREG
	unspec.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r251555