[mlir][StandardToSPIRV] Add support for lowering math.powf to SPIR-V.

[hanchung]

I think we should have a cleanup someday... many ops are no longer Standard ops...

[antiagainst]

In D100403#2686550, @hanchung wrote:

I think we should have a cleanup someday... many ops are no longer Standard ops...

Yup. Need to split out the code for memref stuff at least. Those load/store op handling spans quite some lines. Extracting into another file is better.

mlir/lib/Conversion/StandardToSPIRV/StandardToSPIRV.cpp

void populateStandardToSPIRVPatterns(SPIRVTypeConverter &typeConverter,
 
   patterns.add<
       // Math dialect operations.
       // TODO: Move to separate pass.
       UnaryAndBinaryOpPattern<math::CosOp, spirv::GLSLCosOp>,
       UnaryAndBinaryOpPattern<math::ExpOp, spirv::GLSLExpOp>,
       UnaryAndBinaryOpPattern<math::LogOp, spirv::GLSLLogOp>,
       UnaryAndBinaryOpPattern<math::RsqrtOp, spirv::GLSLInverseSqrtOp>,
+      UnaryAndBinaryOpPattern<math::PowFOp, spirv::GLSLPowOp>,
       UnaryAndBinaryOpPattern<math::SinOp, spirv::GLSLSinOp>,
       UnaryAndBinaryOpPattern<math::SqrtOp, spirv::GLSLSqrtOp>,
       UnaryAndBinaryOpPattern<math::TanhOp, spirv::GLSLTanhOp>,
 
       // Unary and binary patterns
       BitwiseOpPattern<AndOp, spirv::LogicalAndOp, spirv::BitwiseAndOp>,
       BitwiseOpPattern<OrOp, spirv::LogicalOrOp, spirv::BitwiseOrOp>,
       UnaryAndBinaryOpPattern<AbsFOp, spirv::GLSLFAbsOp>,

mlir/test/Conversion/StandardToSPIRV/std-ops-to-spirv.mlir

func @float32_binary_scalar(%lhs: f32, %rhs: f32) {
   // CHECK: spv.FSub %{{.*}}, %{{.*}}: f32
   %1 = subf %lhs, %rhs: f32
   // CHECK: spv.FMul %{{.*}}, %{{.*}}: f32
   %2 = mulf %lhs, %rhs: f32
   // CHECK: spv.FDiv %{{.*}}, %{{.*}}: f32
   %3 = divf %lhs, %rhs: f32
   // CHECK: spv.FRem %{{.*}}, %{{.*}}: f32
   %4 = remf %lhs, %rhs: f32
+  // CHECK: spv.GLSL.Pow %{{.*}}: f32
+  %5 = math.powf %lhs, %rhs : f32
   return
 }
 
 // Check int vector types.
 // CHECK-LABEL: @int_vector234
 func @int_vector234(%arg0: vector<2xi8>, %arg1: vector<4xi64>) {
   // CHECK: spv.SDiv %{{.*}}, %{{.*}}: vector<2xi8>
   %0 = divi_signed %arg0, %arg0: vector<2xi8>