[PowerPC] Fix transform in table gen file causing UB

Running a bootstrap build with UBSan produces a number of instances where
we have signed integer overflow due to this transform. Change the type to
long to prevent this UB on 64-bit build machines.

llvm-svn: 325347

llvm/lib/Target/PowerPC/PPCInstrInfo.td

@@ -308,7 +308,7 @@ def HI16 : SDNodeXForm<imm, [{
 
 def HA16 : SDNodeXForm<imm, [{
   // Transformation function: shift the immediate value down into the low bits.
-  int Val = N->getZExtValue();
+  long Val = N->getZExtValue();
   return getI32Imm((Val - (signed short)Val) >> 16, SDLoc(N));
 }]>;
 def MB : SDNodeXForm<imm, [{