[SystemZ] Fix/optimize vec_load_len and related intrinsics

When using vec_load/store_len_r with an immediate length operand
of 16 or larger, LLVM will currently emit an VLRL/VSTRL instruction
with that immediate.  This creates a valid encoding (which should be
supported by the assembler), but always traps at runtime.  This patch
fixes this by not creating VLRL/VSTRL in those cases.

This would result in loading the length into a register and
calling VLRLR/VSTRLR instead.  However, these operations with
a length of 15 or larger are in fact simply equivalent to a
full vector load or store.  And in fact the same holds true for
vec_load/store_len as well.

Therefore, add a DAGCombine rule to replace those operations with
plain vector loads or stores if the length is known at compile
time and equal or larger to 15.