[RISCV] Adjust select shuffle cost to reflect mask creation cost (#77963

)

This is inspired by
#77342 (review),
and is split off of same with some differences in style.

A select is a vmerge.vv with the additional cost of materializing the
bitmask vector in a vreg. All masks fit within a single vector register
(e8 + m8 is the worst case), and thus our worst case cost should be
roughly 3 (2 scalar to produce the address, one vector load op). Given
most shuffles are small, and the mask will be instead produced by
LUI/ADDI + vmv.s.x or ADDI + vmv.s.x, using 2 as the default seems quite
reasonable. At worst, we're not going to be off by much.

The prior lowering scaled the cost of the bitmask with LMUL, which I
don't understand. At m1 it did use the same base cost of 2. (@lukel97
You wrote the original code here, anything I'm missing here?)

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -84,8 +84,6 @@ RISCVTTIImpl::getRISCVInstructionCost(ArrayRef<unsigned> OpCodes, MVT VT,
       Cost += VL;
       break;
     }
-    case RISCV::VMV_S_X:
-      // FIXME: VMV_S_X doesn't use LMUL, the cost should be 1
     default:
       Cost += LMULCost;
     }
@@ -443,10 +441,13 @@ InstructionCost RISCVTTIImpl::getShuffleCost(TTI::ShuffleKind Kind,
     // vsetivli     zero, 8, e8, mf2, ta, ma (ignored)
     // vmv.s.x      v0, a0
     // vmerge.vvm   v8, v9, v8, v0
+    // We use 2 for the cost of the mask materialization as this is the true
+    // cost for small masks and most shuffles are small.  At worst, this cost
+    // should be a very small constant for the constant pool load.  As such,
+    // we may bias towards large selects slightly more than truely warranted.
     return LT.first *
-           (TLI->getLMULCost(LT.second) + // FIXME: should be 1 for li
-            getRISCVInstructionCost({RISCV::VMV_S_X, RISCV::VMERGE_VVM},
-                                    LT.second, CostKind));
+           (2 + getRISCVInstructionCost({RISCV::VMERGE_VVM},
+                                        LT.second, CostKind));
   }
   case TTI::SK_Broadcast: {
     bool HasScalar = (Args.size() > 0) && (Operator::getOpcode(Args[0]) ==