This CL also changes UnimplementedLoweringError to display the name of
the unimplemented instruction.

Improve test coverage for ARM32 vector load instructions.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4076
R=jpp@chromium.org, kschimpf@google.com

Review URL: https://codereview.chromium.org/1639923002 .