6663621: JVM crashes while trying to execute api/java_security/Signature/SignatureTests.html#initSign tests.
Summary: alignment expression with secondary induction variables is sometimes wrong
Reviewed-by:

Superword uses the pre loop to align the index variable so that it can
use memory operations of larger alignment than the original. The
expression doesn't work correctly when the primary induction variable
strides in a different direction than the secondary induction used in
the memory access. This is actually broken on all platforms but sparc
64-bit was the only one which faulted on the access. The other
platforms were silently fixing up the memory operation resulting in
reduced performance. For some examples it was 30x slower after
vectorization because of this.

The fix is to derive and emit the correct limit expression for these
cases. They are simply slight variations on the existing expressions,
differing only in the sign of an operation. I also took the
opportunity the expand the comments describing the expressions and to
make the comments and source use similar naming based on a lot of
feedback from Chuck.