Fix AArch64 ABI conformance issue in SIMD code

Fix AArch64 ABI conformance issue in SIMD code

In the AArch64 ABI, the high (unused) DWORD of a 32-bit argument's
register is undefined, so it was incorrect to use 64-bit
instructions to transfer a JDIMENSION argument in the 64-bit NEON SIMD
functions.  The code worked thus far only because the existing compiler
optimizers weren't smart enough to do anything else with the register in
question, so the upper 32 bits happened to be all zeroes.

The latest builds of Clang/LLVM have a smarter optimizer, and under
certain circumstances, it will attempt to load-combine adjacent 32-bit
integers from one of the libjpeg structures into a single 64-bit integer
and pass that 64-bit integer as a 32-bit argument to one of the SIMD
functions (which is allowed by the ABI, since the upper 32 bits of the
32-bit argument's register are undefined.)  This caused the
libjpeg-turbo regression tests to crash.

This patch tries to use the Wn registers whenever possible.  Otherwise,
it uses a zero-extend instruction to avoid using the upper 32 bits of
the 64-bit registers, which are not guaranteed to be valid for 32-bit
arguments.

Based on https://github.com/sebpop/libjpeg-turbo/commit/1fbae13021eb98f6fffdfaf8678fcdb00b0b04d9

Closes #91.  Refer also to android-ndk/ndk#110 and
https://llvm.org/bugs/show_bug.cgi?id=28393