[PowerPC] Implement the isSelectSupported() target hook

Summary:
PowerPC has scalar selects (isel) and vector mask selects (xxsel). But PowerPC
does not have vector CR selects, PowerPC does not support scalar condition 
selects on vectors.
In addition to implementing this hook, isSelectSupported() should return false
when the SelectSupportKind is ScalarCondVectorVal, so that predictable selects
are converted into branch sequences.

Reviewed By: steven.zhang,  hfinkel

Differential Revision: https://reviews.llvm.org/D55754

llvm-svn: 349727