tint/hlsl: for default-only switch, only emit condition if it has side-effects

This fixes edge-cases, like the condition expression being a type-cast,
which DXC apparently sees as a variable re-declaration. Example:

fn foo(x : f32) {
  switch (i32(x)) {
    default {
    }
  }
}

was emitted as HLSL:

void foo(float x) {
  int(x);
  do {
  } while (false);
}

The `int(x)` is seen as a re-declaration of `x` by DXC.

We fix this by only emitted the condition expression if it has
side-effects (which currently means it contains a call expression).

Bug: tint:1820
Change-Id: I7e4320fa09ea2d634c9e324cb0b752b0ee7dcde9
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/118161
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>