[PowerPC] support register pressure reduction in machine combiner.

Reassociating some patterns to generate more fma instructions to
reduce register pressure.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D92071

Author: Chen Zheng

llvm/include/llvm/CodeGen/MachineCombinerPattern.h

@@ -29,6 +29,11 @@ enum class MachineCombinerPattern {
   REASSOC_XY_AMM_BMM,
   REASSOC_XMM_AMM_BMM,
 
+  // These are patterns matched by the PowerPC to reassociate FMA and FSUB to
+  // reduce register pressure.
+  REASSOC_XY_BCA,
+  REASSOC_XY_BAC,
+
   // These are multiply-add patterns matched by the AArch64 machine combiner.
   MULADDW_OP1,
   MULADDW_OP2,

llvm/lib/CodeGen/MachineCombiner.cpp

@@ -279,6 +279,9 @@ static CombinerObjective getCombinerObjective(MachineCombinerPattern P) {
   case MachineCombinerPattern::REASSOC_XY_AMM_BMM:
   case MachineCombinerPattern::REASSOC_XMM_AMM_BMM:
     return CombinerObjective::MustReduceDepth;
+  case MachineCombinerPattern::REASSOC_XY_BCA:
+  case MachineCombinerPattern::REASSOC_XY_BAC:
+    return CombinerObjective::MustReduceRegisterPressure;
   default:
     return CombinerObjective::Default;
   }