[SystemZ] Fix internal error with single-element vector types (#196127)
The special treatment of single-element 128-bit vector types in
SystemZTargetLowering::getRegisterTypeForCallingConv is not appropriate
if vector types are not supported, and can lead to internal compiler
errors.
Fixes: https://github.com/llvm/llvm-project/issues/194256
(cherry picked from commit 48346f2352eaf25373e1a6204c0c7f9fdce92a85)