proper fp8 types lowering from ml_dtypes

[rrrrice]

example mlir

module {
  func.func @extend_f8_to_f32(%arg0: f8E5M2) -> f32 {
    %0 = arith.extf %arg0 : f8E5M2 to f32
    return %0 : f32
  }
  func.func @truncate_f32_to_f8(%arg0: f32) -> f8E5M2 {
    %0 = arith.truncf %arg0 : f32 to f8E5M2
    return %0 : f8E5M2
  }
}

fp8 gets lowered into i8

backend = LLVMJITBackend()
compiled = backend.compile(
    module_finished,
    kernel_name="extend_f8_to_f32",
    pipeline=Pipeline().lower_to_llvm(),
)
invoker = backend.load(compiled)
input_val = ml_dtypes.float8_e5m2(0.5)
result = invoker.extend_f8_to_f32(input_val)

mlir.extras.runtime.passes.MlirCompilerError: Lowering IR failed with the following diagnostics:

********************************************************************************
Failure while executing pass pipeline:
error: unknown: 'llvm.fpext' op operand #0 must be floating point LLVM type or LLVM dialect-compatible vector of floating point LLVM type, but got 'i8'
note: unknown: see current operation: %1 = "llvm.fpext"(%arg0) : (i8) -> f32
error: unknown: 'llvm.fptrunc' op result #0 must be floating point LLVM type or LLVM dialect-compatible vector of floating point LLVM type, but got 'i8'
note: unknown: see current operation: %0 = "llvm.fptrunc"(%arg0) : (f32) -> i8
********************************************************************************

[rrrrice]

@makslevental

[llvmbot]

@llvm/issue-subscribers-mlir

Author: None (rrrrice)

example mlir ``` module { func.func @extend_f8_to_f32(%arg0: f8E5M2) -> f32 { %0 = arith.extf %arg0 : f8E5M2 to f32 return %0 : f32 } func.func @truncate_f32_to_f8(%arg0: f32) -> f8E5M2 { %0 = arith.truncf %arg0 : f32 to f8E5M2 return %0 : f8E5M2 } } ```

fp8 gets lowered into i8

backend = LLVMJITBackend()
compiled = backend.compile(
    module_finished,
    kernel_name="extend_f8_to_f32",
    pipeline=Pipeline().lower_to_llvm(),
)
invoker = backend.load(compiled)
input_val = ml_dtypes.float8_e5m2(0.5)
result = invoker.extend_f8_to_f32(input_val)

mlir.extras.runtime.passes.MlirCompilerError: Lowering IR failed with the following diagnostics:

********************************************************************************
Failure while executing pass pipeline:
error: unknown: 'llvm.fpext' op operand #<!-- -->0 must be floating point LLVM type or LLVM dialect-compatible vector of floating point LLVM type, but got 'i8'
note: unknown: see current operation: %1 = "llvm.fpext"(%arg0) : (i8) -&gt; f32
error: unknown: 'llvm.fptrunc' op result #<!-- -->0 must be floating point LLVM type or LLVM dialect-compatible vector of floating point LLVM type, but got 'i8'
note: unknown: see current operation: %0 = "llvm.fptrunc"(%arg0) : (f32) -&gt; i8
********************************************************************************