GlobalISel: Add simpler way of always specifying custom lowering

[paquette]

LGTM

[arsenm]

r352637

include/llvm/CodeGen/GlobalISel/LegalizerInfo.h

LegalizeRuleSet &customForCartesianProduct(std::initializer_list<LLT> Types) {
     return actionForCartesianProduct(LegalizeAction::Custom, Types);
   }
   LegalizeRuleSet &
   customForCartesianProduct(std::initializer_list<LLT> Types0,
                             std::initializer_list<LLT> Types1) {
     return actionForCartesianProduct(LegalizeAction::Custom, Types0, Types1);
   }
 
+  /// Unconditionally custom lower.
+  LegalizeRuleSet &custom() {
+    return customIf(always);
+  }
+
   /// Widen the scalar to the next power of two that is at least MinSize.
   /// No effect if the type is not a scalar or is a power of two.
   LegalizeRuleSet &widenScalarToNextPow2(unsigned TypeIdx,
                                          unsigned MinSize = 0) {
     using namespace LegalityPredicates;
     return actionIf(LegalizeAction::WidenScalar, sizeNotPow2(typeIdx(TypeIdx)),
                     LegalizeMutations::widenScalarToNextPow2(TypeIdx, MinSize));
   }