[Sema] Use CK_NoOp instead CK_Invalid in tryGCCVectorConvertAndSplat

This fix UBSAN bots after r302935. Storing non-defined values in enum is
undefined behavior.

Other places, where "if (ScalarCast != CK_Invalid)" is used, never get to the
"if" with CK_Invalid. tryGCCVectorConvertAndSplat can get to the "if" with
CK_Invalid and it looks like expected case. So we have to use something other
than CK_Invalid, e.g. CK_NoOp.

llvm-svn: 303121