[SystemZ] Fix build bot failures

My patch 9db13b5a seems to have
caused some build bots to fail due to warnings that appear only
when using -Wcovered-switch-default.

This patch is an attempt to fix this by trying to avoid both the warning
"default label in switch which covers all enumeration values"
for the inner switch statements and at the same time the warning
"this statement may fall through"
for the outer switch statement in getVectorComparison
(SystemZISelLowering.cpp).