[AArch64] Make simd_immediate_info INDEX explicit

[AArch64] Make simd_immediate_info INDEX explicit

This patch tweaks the representation of SVE INDEX instructions in
simd_immediate_info so that it's easier to add new types of
constant on top.

2019-08-13  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (simd_immediate_info::insn_type): Add
	INDEX.
	(simd_immediate_info::value, simd_immediate_info::step)
	(simd_immediate_info::modifier, simd_immediate_info::shift): Replace
	with...
	(simd_immediate_info::u): ...this new union.
	(simd_immediate_info::simd_immediate_info): Update accordingly.
	(aarch64_output_simd_mov_immediate): Likewise.
	(aarch64_output_sve_mov_immediate): Likewise.

From-SVN: r274371