PPC: Teach FMA mutate to respect register classes.

This was causing bad code gen and assembly that won't assemble, as
mixed altivec and vsx code would end up with a vsx high register
assigned to an altivec instruction, which won't work. Constraining the
classes allows the optimization to proceed.

llvm-svn: 255299

lib/Target/PowerPC/PPCVSXFMAMutate.cpp

@@ -220,6 +220,14 @@ protected:
         if (OldFMAReg == KilledProdReg)
           continue;
 
+        // If there isn't a class that fits, we can't perform the transform.
+        // This is needed for correctness with a mixture of VSX and Altivec
+        // instructions to make sure that a low VSX register is not assigned to
+        // the Altivec instruction.
+        if (!MRI.constrainRegClass(KilledProdReg,
+                                   MRI.getRegClass(OldFMAReg)))
+          continue;
+
         assert(OldFMAReg == AddendMI->getOperand(0).getReg() &&
                "Addend copy not tied to old FMA output!");
 
@@ -262,8 +270,7 @@ protected:
           if (UseMI == AddendMI)
             continue;
 
-          UseMO.setReg(KilledProdReg);
-          UseMO.setSubReg(KilledProdSubReg);
+          UseMO.substVirtReg(KilledProdReg, KilledProdSubReg, *TRI);
         }
 
         // Extend the live intervals of the killed product operand to hold the