stage2: delete astgen for switch expressions

The astgen for switch expressions did not respect the ZIR rules of only
referencing instructions that are in scope:

  %14 = block_comptime_flat({
    %15 = block_comptime_flat({
      %16 = const(TypedValue{ .ty = comptime_int, .val = 1})
    })
    %17 = block_comptime_flat({
      %18 = const(TypedValue{ .ty = comptime_int, .val = 2})
    })
  })
  %19 = block({
    %20 = ref(%5)
    %21 = deref(%20)
    %22 = switchbr(%20, [%15, %17], {
      %15 => {
        %23 = const(TypedValue{ .ty = comptime_int, .val = 1})
        %24 = store(%10, %23)
        %25 = const(TypedValue{ .ty = void, .val = {}})
        %26 = break("label_19", %25)
      },
      %17 => {
        %27 = const(TypedValue{ .ty = comptime_int, .val = 2})
        %28 = store(%10, %27)
        %29 = const(TypedValue{ .ty = void, .val = {}})
        %30 = break("label_19", %29)
      }
    }, {
      %31 = unreachable_safe()
    }, special_prong=else)
  })

In this snippet you can see that the comptime expr referenced %15 and
%17 which are not in scope. There also was no test coverage for runtime
switch expressions.

Switch expressions will have to be re-introduced to follow these rules
and with some test coverage. There is some usable code being deleted in
this commit; it will be useful to reference when re-implementing switch
later.

A few more improvements to do while we're at it:
 * only use .ref result loc on switch target if any prongs obtain the
   payload with |*syntax|
   - this improvement should be done to if, while, and for as well.
   - this will remove the needless ref/deref instructions above
 * remove switchbr and add switch_block, which is both a block and a
   switch branch.
   - similarly we should remove loop and add loop_block.

This commit introduces a "force_comptime" flag into the GenZIR
scope. The main purpose of this will be to choose the "comptime"
variants of certain key zir instructions, such as function calls and
branches. We will be moving away from using the block_comptime_flat
ZIR instruction, and eventually deleting it.

This commit also contains miscellaneous fixes to this branch that bring
it to the state of passing all the tests.

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