GlobalISel: select G_VASTART on iOS AArch64.

The AAPCS ABI is substantially more complicated so that's coming in a separate
patch. For now we can generate correct code for iOS though.

llvm-svn: 294493
diff --git a/llvm/lib/Target/AArch64/AArch64LegalizerInfo.cpp b/llvm/lib/Target/AArch64/AArch64LegalizerInfo.cpp
index 3a97406..ad0482a 100644
--- a/llvm/lib/Target/AArch64/AArch64LegalizerInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64LegalizerInfo.cpp
@@ -225,5 +225,7 @@
     setAction({G_BITCAST, 1, LLT::vector(32/EltSize, EltSize)}, Legal);
   }
 
+  setAction({G_VASTART, p0}, Legal);
+
   computeTables();
 }