[CodeGen] Make the code that detects a if a shuffle is really a concatenation...

[CodeGen] Make the code that detects a if a shuffle is really a concatenation of the inputs more general purpose.

We can now handle concatenation of each source multiple times. The previous code just checked for each source to appear once in either order.

This also now handles an entire source vector sized piece having undef indices correctly. We now concat with UNDEF instead of using one of the sources. This is responsible for the test case change.

llvm-svn: 274483