[ROCM] remove fma dispatch (#591)

* removed fma dispatch

* added comments to explain why remove fma

* fix lint

* use fmuladd intrin for fma dispatch

src/codegen/llvm/intrin_rule_rocm.cc

@@ -29,8 +29,10 @@ namespace llvm {
 TVM_REGISTER_GLOBAL("tvm.intrin.rule.rocm.exp")
 .set_body(DispatchExternOCML);
 
+// On AMD GPU, fma is slower than mac
+// removing fma dispatch allows backend to generate faster mac instruction
 TVM_REGISTER_GLOBAL("tvm.intrin.rule.rocm.fma")
-.set_body(DispatchExternOCML);
+.set_body(DispatchLLVMPureIntrin<::llvm::Intrinsic::fmuladd, 1>);
 
 TVM_REGISTER_GLOBAL("tvm.intrin.rule.rocm.log")
 .set_body(DispatchExternOCML);