[SystemZ] Support vector load/store alignment hints

Vector load/store instructions support an optional alignment field
that the compiler can use to provide known alignment info to the
hardware.  If the field is used (and the information is correct),
the hardware may be able (on some models) to perform faster memory
accesses than otherwise.

This patch adds support for alignment hints in the assembler and
disassembler, and fills in known alignment during codegen.

llvm-svn: 363806