[LinxISA] extend v0.4 codegen runtime coverage

[zhoubot]

Summary

• extend Linx v0.4 codegen, frame, MC, and SIMT autovec handling for the current runtime bring-up path
• add focused Linx CodeGen/MC coverage for local labels, p-save/restore canonicalization, vblock launch rejection, and new SIMT autovec cases
• tighten two new tests to the currently implemented contract (unsupported_exit_side_effects rejection and relaxed if-conversion shape checks)

Validation

• cd /Users/zhoubot/linx-isa/compiler/llvm/build-linxisa-clang && ninja llc llvm-mc
• targeted Linx codegen/MC checks using llc, clang, llvm-mc, llvm-objdump, split-file, and FileCheck over:
  • dynamic-stackalloc.ll, stacksave-expand.ll, vblock-launch.ll, autovec_reduction_liveout.ll
  • autovec_active_replay_break.ll, autovec_ifconverted_diamond.ll, autovec_layout_policy.ll, autovec_nested_mask_branch.ll
  • blockify-setc-imm-liveout.mir, vblock-launch-reject-archived-base.ll
  • body-local-label-branches.s, simt-p-save-restore-gap.s, simt-p-save-restore-roundtrip.s, v03-vector-reuse-roundtrip.s