[AArch64] Fix handling of npatterns>1 constants for partial SVE modes

[AArch64] Fix handling of npatterns>1 constants for partial SVE modes

For partial SVE vectors of element X, we want to treat duplicates
of single X elements in the same way as for full vectors of X.
But if a constant instead contains a repeating pattern of X elements,
the transition from one value to the next must happen at container
boundaries rather than element boundaries.  E.g. a VNx4HI should
in that case contain the same number of constants as a VNx4SI.

Fixing this means that we need a reinterpret from the container-based
mode to the partial mode; e.g. in the above example we need a
reinterpret from VNx4SI to VNx4HI.  We can't use subregs for that
because they're forbidden by aarch64_can_change_class_mode; we should
handle them in the same way as for big-endian instead.

2019-12-19  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_simd_valid_immediate): When
	handling partial SVE vectors, use the container mode rather than
	the element mode if the constant isn't a single-element duplicate.
	* config/aarch64/aarch64-sve.md (@aarch64_sve_reinterpret<mode>):
	Check targetm.can_change_mode_class instead of BYTES_BIG_ENDIAN.

gcc/testsuite/
	* gcc.target/aarch64/sve/mixed_size_9.c: New test.

From-SVN: r279580