Reformat xlang-ir to use a new basic block design

[chorman0773]

XLang IR Changes:

• Targets are removed, replaced with basic blocks, which contain a list of expressions followed by exactly one terminator
  • Basic Block entries still have an incoming stack
• Removed expressions that diverge in favour of terminators for basic blocks
  • Removed call, tailcall, exit, asm, branch, branch indirect, and switch expressions
• Added terminators for basic blocks:
  • Added call and tailcall terminators. In addition to the old syntax, the call terminator accepts a jump-info (target + optional flags) for a "next" block to jump to after the call finishes. tailcall omits next, call requires it.
  • Added exit terminator
  • Added jump terminator which takes a jump-info, and is used for unconditional jumps
  • Added a branch terminator for conditional branches. These take two jump-infos, one when the condition is satisfied, and one when it is not. never and always continue to exist, and act like jumps to the respective targets, but also typecheck the untaken branch.
  • Added jump indirect for indirect jumps.
  • Added asm terminator, which additionally takes a next jump-info to continue to unless the block is noreturn.
  • Added switch terminator.
  • Added new unreachable terminator, which should be favoured over const undef invalid T; exit 1. const undef invalid can still be used when a value is statically needed by the code generator.

Also changed the structure of xlang-backend to handle the new input correctly.

[rdrpenguin04]

I'll just approve this