stablehlo.iota fails for f16 on vmvx backend.

[bviyer]

What happened?

For the following function:

func.func @iota_dim1_f16() {
    %result = "stablehlo.iota"() {iota_dimension = 1 : i64} : () -> tensor<2x3xf16>
    check.expect_almost_eq_const(%result, dense<[
      [0.0, 1.0, 2.0],
      [0.0, 1.0, 2.0]]> : tensor<2x3xf16>) : tensor<2x3xf16>
    return
  }

When compiled with the following command:

./tools/iree-compile /tmp/iota_iree.mlir --iree-hal-target-backends=vmvx

Fails with the following issue:

/tmp/iota_iree.mlir:17:15: error: failed to legalize operation 'arith.sitofp' that was explicitly marked illegal
    %result = "stablehlo.iota"() {iota_dimension = 0 : i64} : () -> tensor<2x3xf16>
              ^
/tmp/iota_iree.mlir:17:15: note: see current operation: %29 = "arith.sitofp"(%28) : (i16) -> f16
/tmp/iota_iree.mlir:17:15: error: conversion to vm.module failed
    %result = "stablehlo.iota"() {iota_dimension = 0 : i64} : () -> tensor<2x3xf16>
              ^

Steps to reproduce your issue

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What component(s) does this issue relate to?

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Version information

HEAD pointed to: 4a80ee36d52b35be25bf30ea6a0f9b423477f6e9

Additional context

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