ir_convert should recognize inline array as valid RHS const expr in proc config functions

[proppy]

Describe the bug
In the context of a proc config function, it seems that ir_convert doesn't recognize inline array definitions as const expr .

To Reproduce

Attempting to convert the following proc:

import float32;
type F32 = float32::F32;

const F32_0 = float32::zero(false);
const F32_2 = F32 { sign: false, bexp: u8:128, fraction: u23:0 };

proc matmul {
    weight: F32[u32:4][u32:4];
    config() {
      let weights = F32[u32:4][u32:4]:[
          [F32_2, F32_0, F32_0, F32_0], [F32_0, F32_2, F32_0, F32_0], [F32_0, F32_0, F32_2, F32_0],
          [F32_0, F32_0, F32_0, F32_2],
      ];
      (weights,)
    }

    init { () }

    next(state: ()) {
      ()
    }
}

will produce the following error:

Error: INTERNAL: Let RHS not evaluated as constexpr: weights : F32[u32:4][u32:4]:[[F32_2, F32_0, F32_0, F32_0], [F32_0, F32_2, F32_0, F32_0], [F32_0, F32_0, F32_2, F32_0], [F32_0, F32_0, F32_0, F32_2]]

Expected behavior
ir_convert infer the const expr-ness for the array let binding.

[proppy]

Note that switching let weights to const WEIGHTS doesn't seems to help.

[proppy]

And moving it outside of config triggers #1747 (comment).