GlobalISel: Scalarize all division operations

This only handled G_SDIV, but they all are trivially scalarizable.

Also define placeholder AMDGPU division legalizer rules.
diff --git a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
index 0a792dd..52c43f0 100644
--- a/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/LegalizerHelper.cpp
@@ -3021,6 +3021,9 @@
   case G_BSWAP:
   case G_BITREVERSE:
   case G_SDIV:
+  case G_UDIV:
+  case G_SREM:
+  case G_UREM:
   case G_SMIN:
   case G_SMAX:
   case G_UMIN: