[SystemZ] Add support for z13 low-level vector builtins

This adds low-level builtins to allow access to all of the z13 vector
instructions.  Note that instructions whose semantics can be described
by standard C (including clang extensions) do not get any builtins.

For each instructions whose semantics *cannot* (fully) be described, we
define a builtin named __builtin_s390_<insn> that directly maps to this
instruction.  These are intended to be compatible with GCC.

For instructions that also set the condition code, the builtin will take
an extra argument of type "int *" at the end.  The integer pointed to by
this argument will be set to the post-instruction CC value.

For many instructions, the low-level builtin is mapped to the corresponding
LLVM IR intrinsic.  However, a number of instructions can be represented
in standard LLVM IR without requiring use of a target intrinsic.

Some instructions require immediate integer operands within a certain
range.  Those are verified at the Sema level.

Based on a patch by Richard Sandiford.

llvm-svn: 236532