[WebAssembly] Handle errors in getRegFor{Uns,S}ignedValue.

[sunfish]

Summary

This fixes bug 35064, though it needs a reduced testcase still.

[jbhateja]

ConstantExpr are not handled by FastISel.

Similar checks should be added inside WebAssemblyFastISel::selectSExt and WebAssemblyFastISel::selectZExt

Thanks.

[sunfish]

Fixed in r319144, along with selectSExt and selectZExt, and I fixed a few other places that needed similar changes as well. Thanks!

lib/Target/WebAssembly/WebAssemblyFastISel.cpp

 unsigned WebAssemblyFastISel::getRegForUnsignedValue(const Value *V) {
   MVT::SimpleValueType From = getSimpleType(V->getType());
   MVT::SimpleValueType To = getLegalType(From);
-  return zeroExtend(getRegForValue(V), V, From, To);
+  unsigned VReg = getRegForValue(V);
+  if (VReg == 0)
+    return 0;
+  return zeroExtend(VReg, V, From, To);
 }
 
 unsigned WebAssemblyFastISel::getRegForSignedValue(const Value *V) {
   MVT::SimpleValueType From = getSimpleType(V->getType());
   MVT::SimpleValueType To = getLegalType(From);
-  return zeroExtend(getRegForValue(V), V, From, To);
+  unsigned VReg = getRegForValue(V);
+  if (VReg == 0)
+    return 0;
+  return signExtend(VReg, V, From, To);
 }
 
 unsigned WebAssemblyFastISel::getRegForPromotedValue(const Value *V,