[GlobalISel][AArch64] Generate ptrtoint/inttoptr as opposed to bitcast in unmerge combine.

When combining unmerge we could end up with ptr to i64 bitcasts. Make sure they
are created as ptrtoint/inttoptr instead.

Some of the test are still disabled as they hit other issues.

Author: davemgreen

llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h

@@ -1218,8 +1218,14 @@ class LegalizationArtifactCombiner {
     } else {
       LLT MergeSrcTy = MRI.getType(MergeI->getOperand(1).getReg());
 
-      if (!ConvertOp && DestTy != MergeSrcTy)
-        ConvertOp = TargetOpcode::G_BITCAST;
+      if (!ConvertOp && DestTy != MergeSrcTy) {
+        if (DestTy.isPointer())
+          ConvertOp = TargetOpcode::G_INTTOPTR;
+        else if (MergeSrcTy.isPointer())
+          ConvertOp = TargetOpcode::G_PTRTOINT;
+        else
+          ConvertOp = TargetOpcode::G_BITCAST;
+      }
 
       if (ConvertOp) {
         Builder.setInstr(MI);