[PowerPC] Avoid VSX FMA mutate when killed product reg = addend reg

With VSX enabled, test/CodeGen/PowerPC/recipest.ll exposes a bug in
the FMA mutation pass.  If we have a situation where a killed product
register is the same register as the FMA target, such as:

   %vreg5<def,tied1> = XSNMSUBADP %vreg5<tied0>, %vreg11, %vreg5,
                       %RM<imp-use>; VSFRC:%vreg5 F8RC:%vreg11 

then the substitution makes no sense.  We end up getting a crash when
we try to extend the interval associated with the killed product
register, as there is already a live range for %vreg5 there.  This
patch just disables the mutation under those circumstances.

Since recipest.ll generates different code with VMX enabled, I've
modified that test to use -mattr=-vsx.  I've borrowed the code from
that test that exposed the bug and placed it in fma-mutate.ll, where
it tests several mutation opportunities including the "bad" one.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220290 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCInstrInfo.cpp

@@ -1754,6 +1754,12 @@ protected:
 
         unsigned OldFMAReg = MI->getOperand(0).getReg();
 
+        // The transformation doesn't work well with things like:
+        //    %vreg5 = A-form-op %vreg5, %vreg11, %vreg5;
+        // so leave such things alone.
+        if (OldFMAReg == KilledProdReg)
+          continue;
+
         assert(OldFMAReg == AddendMI->getOperand(0).getReg() &&
                "Addend copy not tied to old FMA output!");