[M68k] Implement TLI::convertSelectOfConstantsToMath

This TLI callback shows the preference of other operations over
ISD::SELECT on constant operands. It's essential for M68k because
ISD::SELECT tends to lower into M68kISD::CMOV which is not always ideal.

This fixes #130754, which showed up after 352c48f.

Author: mshockwave

llvm/lib/Target/M68k/M68kISelLowering.h

@@ -190,6 +190,10 @@ class M68kTargetLowering : public TargetLowering {
   InlineAsm::ConstraintCode
   getInlineAsmMemConstraint(StringRef ConstraintCode) const override;
 
+  // We need this for DAGCombiner to eliminate as many ISD::SELECT as possible.
+  // Otherwise we might end up with M68kISD::CMOV.
+  bool convertSelectOfConstantsToMath(EVT VT) const override { return true; }
+
 private:
   unsigned GetAlignedArgumentStackSize(unsigned StackSize,
                                        SelectionDAG &DAG) const;