[SystemZ] Add vector intrinsics

This adds intrinsics to allow access to all of the z13 vector instructions.
Note that instructions whose semantics can be described by standard LLVM IR
do not get any intrinsics.

For each instructions whose semantics *cannot* (fully) be described, we
define an LLVM IR target-specific intrinsic that directly maps to this
instruction.

For instructions that also set the condition code, the LLVM IR intrinsic
returns the post-instruction CC value as a second result.  Instruction
selection will attempt to detect code that compares that CC value against
constants and use the condition code directly instead.

Based on a patch by Richard Sandiford.

llvm-svn: 236527