[SystemZ] Only attempt RxSBG optimization for integer types

As pointed out by Justin Bogner (see r240520), SystemZDAGToDAGISel::Select
currently attempts to convert boolean operations into RxSBG even on some
non-integer types (in particular, vector types).  This would not work in
any case, and it happened to trigger undefined behaviour in allOnes.

This patch verifies that we have a (<= 64-bit) integer type before
attempting to perform this optimization.

llvm-svn: 240634