[CostModel][X86] Fix fpext conversion cost for 16 elements (#76278)

The fpext conversion cost for 16 elements should be 4 from Znver4.

Author: HaohaiWen

llvm/lib/Target/X86/X86TargetTransformInfo.cpp

@@ -2232,6 +2232,7 @@ InstructionCost X86TTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
   static const TypeConversionCostTblEntry AVX512FConversionTbl[] = {
     { ISD::FP_EXTEND, MVT::v8f64,   MVT::v8f32,  1 },
     { ISD::FP_EXTEND, MVT::v8f64,   MVT::v16f32, 3 },
+    { ISD::FP_EXTEND, MVT::v16f64,  MVT::v16f32, 4 }, // 2*vcvtps2pd+vextractf64x4
     { ISD::FP_ROUND,  MVT::v8f32,   MVT::v8f64,  1 },
 
     { ISD::TRUNCATE,  MVT::v2i1,    MVT::v2i8,   3 }, // sext+vpslld+vptestmd

llvm/test/Analysis/CostModel/X86/cast.ll

@@ -632,7 +632,7 @@ define void @fp_conv(<8 x float> %a, <16 x float>%b, <4 x float> %c) {
 ; AVX512-LABEL: 'fp_conv'
 ; AVX512-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %A1 = fpext <4 x float> %c to <4 x double>
 ; AVX512-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %A2 = fpext <8 x float> %a to <8 x double>
-; AVX512-NEXT:  Cost Model: Found an estimated cost of 6 for instruction: %A3 = fpext <16 x float> %b to <16 x double>
+; AVX512-NEXT:  Cost Model: Found an estimated cost of 4 for instruction: %A3 = fpext <16 x float> %b to <16 x double>
 ; AVX512-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %A4 = fptrunc <4 x double> undef to <4 x float>
 ; AVX512-NEXT:  Cost Model: Found an estimated cost of 1 for instruction: %A5 = fptrunc <8 x double> undef to <8 x float>
 ; AVX512-NEXT:  Cost Model: Found an estimated cost of 0 for instruction: ret void