[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