Added missing NEON pcasts, update packetmath tests.

The NEON `pcast` operators are all implemented and tested for existing
packets. This requires adding a `pcast(a,b,c,d,e,f,g,h)` for casting
between `int64_t` and `int8_t` in `GenericPacketMath.h`.

Removed incorrect `HasHalfPacket`  definition for NEON's
`Packet2l`/`Packet2ul`.

Adjustments were also made to the `packetmath` tests. These include
- minor bug fixes for cast tests (i.e. 4:1 casts, only casting for
  packets that are vectorizable)
- added 8:1 cast tests
- random number generation
  - original had uninteresting 0 to 0 casts for many casts between
    floating-point and integers, and exhibited signed overflow
    undefined behavior

Tested:
```
$ aarch64-linux-gnu-g++ -static -I./ '-DEIGEN_TEST_PART_ALL=1' test/packetmath.cpp -o packetmath
$ adb push packetmath /data/local/tmp/
$ adb shell "/data/local/tmp/packetmath"
```

1 files changed, 2 insertions, 2 deletions

diff --git a/Eigen/src/Core/arch/NEON/PacketMath.h b/Eigen/src/Core/arch/NEON/PacketMath.h
index ddd43c377..340e1b1c5 100644
--- a/Eigen/src/Core/arch/NEON/PacketMath.h
+++ b/Eigen/src/Core/arch/NEON/PacketMath.h
@@ -338,7 +338,7 @@ struct packet_traits<int64_t> : default_packet_traits
     Vectorizable = 1,
     AlignedOnScalar = 1,
     size = 2,
-    HasHalfPacket = 1,
+    HasHalfPacket = 0,
 
     HasCmp       = 1,
     HasAdd       = 1,
@@ -368,7 +368,7 @@ struct packet_traits<uint64_t> : default_packet_traits
     Vectorizable = 1,
     AlignedOnScalar = 1,
     size = 2,
-    HasHalfPacket = 1,
+    HasHalfPacket = 0,
 
     HasCmp       = 1,
     HasAdd       = 1,