DAG: Try to custom lower when promoting float operands

[arsenm]

Summary

For some reason this wasn't done for floats like integers.

[craig.topper]

What about PromoteFloatResult? Should it have this too?

[arsenm]

In D50625#1199691, @craig.topper wrote:

What about PromoteFloatResult? Should it have this too?

Probably, though I don't currently have a case that needs it

[craig.topper]

LGTM

[arsenm]

r339811

lib/CodeGen/SelectionDAG/LegalizeFloatTypes.cpp

static ISD::NodeType GetPromotionOpcode(EVT OpVT, EVT RetVT) {
   }
 
   report_fatal_error("Attempt at an invalid promotion-related conversion");
 }
 
 bool DAGTypeLegalizer::PromoteFloatOperand(SDNode *N, unsigned OpNo) {
   SDValue R = SDValue();
 
+  if (CustomLowerNode(N, N->getOperand(OpNo).getValueType(), false)) {
+    LLVM_DEBUG(dbgs() << "Node has been custom lowered, done\n");
+    return false;
+  }
+
   // Nodes that use a promotion-requiring floating point operand, but doesn't
   // produce a promotion-requiring floating point result, need to be legalized
   // to use the promoted float operand.  Nodes that produce at least one
   // promotion-requiring floating point result have their operands legalized as
   // a part of PromoteFloatResult.
   switch (N->getOpcode()) {
     default:
       llvm_unreachable("Do not know how to promote this operator's operand!");