Avoids generating a legalization assert for the case where a vector type is legal

Avoids generating a legalization assert for the case where a vector type is legal
but when legalizing the operation, we split the vector type and generate a library
call whose type needs to be promoted. For example, X86 with SSE on but MMX off,
a divide v2i64 will be scalarized to 2 calls to a library using i64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@63760 91177308-0d34-0410-b5e6-96231b3b80d8