[ARM] Don't try and custom lower a vNi64 SETCC.

[ARM] Don't try and custom lower a vNi64 SETCC.

It won't go well. We've already marked 64-bit SETCCs as non-Custom, but it's just possible that a SETCC has a legal result type but an illegal operand type. If this happens, bail out before we create unselectable nodes.

Fixes PR24292. I tried to create a testcase but in 99% of cases we can't trigger this - not surprising that this bug has been latent since 2009.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245577 91177308-0d34-0410-b5e6-96231b3b80d8