PassMode::Indirect entry inputs should make copies.

[eddyb]

#[spirv(fragment)]
pub fn main(mut xs: [f32; 3]) {
    xs[0] = 0.0;
}

errors with:

error: error:0:0 - OpStore Pointer <id> '15[%15]' storage class is read-only
  OpStore %15 %float_0

  |
  = note: spirv-val failed

This happens because we're passing the Input OpVariable pointer as the pointer for xs.

Crucially, those are the semantics of an Operand::Move call arguments in MIR, transferring ownership from the caller to the callee. That's not correct, the caller doesn't own the global OpVariable, and should make a local copy (in a Function-scoped OpVariable), before passing a pointer to that in the call.

For MIR calls, this is where that's implemented, and we pretty much have to do the same dance (note that OperandValue::Ref(_).store(dest) does a memcpy).