[AArch64] Make aarch64_classify_vector_mode use a switch statement

[AArch64] Make aarch64_classify_vector_mode use a switch statement

aarch64_classify_vector_mode used properties of a mode to test whether
the mode was a single Advanced SIMD vector, a single SVE vector, or a
tuple of SVE vectors.  That works well for current trunk and is simpler
than checking for modes by name.

However, for the ACLE and for planned autovec improvements, we also
need partial SVE vector modes that hold:

- half of the available 32-bit elements
- a half or quarter of the available 16-bit elements
- a half, quarter, or eighth of the available 8-bit elements

These should be packed in memory and unpacked in registers.  E.g.
VNx2SI has half the number of elements of VNx4SI, and so is half the
size in memory.  When stored in registers, each VNx2SI element occupies
the low 32 bits of a VNx2DI element, with the upper bits being undefined.

The upshot is that:

  GET_MODE_SIZE (VNx4SImode) == 2 * GET_MODE_SIZE (VNx2SImode)

since GET_MODE_SIZE must always be the memory size.  This in turn means
that for fixed-length SVE, some partial modes can have the same size as
Advanced SIMD modes.  We then need to be specific about which mode we're
dealing with.

This patch prepares for that by switching based on the mode instead
of querying properties.

A later patch makes sure that Advanced SIMD modes always win over
partial SVE vector modes in normal queries.

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

gcc/
	* config/aarch64/aarch64.c (aarch64_classify_vector_mode): Switch
	based on the mode instead of testing properties of it.

From-SVN: r274368