Don't flag host stack allocations feeding a GPU kernel as cross-talk

[abadams]

Problem

A Func scheduled compute_root().store_in(MemoryType::Stack) and read by a GPU kernel was tripping the GPU register/stack cross-talk check added in #9325, e.g.:

Func input;
input(x) = x * 2;
input.compute_root().store_in(MemoryType::Stack);

Func f;
f(x, y) = input(x) + input(x + 1);
f.gpu_tile(x, y, xo, yo, xi, yi, 8, 8);

fails with:

Error: The allocation f0 is scheduled to live in Stack memory, which is private
to a GPU thread ...

But this allocation lives on the CPU stack — a single host copy that every GPU
thread reads, like any other input buffer — not a per-thread private copy, so
there is no cross-talk to check for.

Cause

CheckCrossTalk::visit(Realize*) flagged any Register/Stack allocation
scheduled outside the loops over GPU threads, without checking whether it was
inside a GPU kernel at all. It couldn't distinguish a genuine block-level
per-thread allocation from a host allocation that merely happens to be read by a
kernel.

Fix

Track whether we are inside any GPU loop (is_gpu(op->for_type)) and only run
the check when the allocation is both inside a kernel and outside the thread
loops.

Test

Added a case to test/correctness/gpu_register_at_block_level.cpp covering a
host stack allocation feeding a GPU kernel. Verified the existing
test/error/gpu_register_* negative tests still correctly reject real
cross-talk.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.06%. Comparing base (e02a03f) to head (5bac27e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9450      +/-   ##
==========================================
+ Coverage   70.03%   70.06%   +0.02%     
==========================================
  Files         261      261              
  Lines       79802    79805       +3     
  Branches    19455    19456       +1     
==========================================
+ Hits        55892    55912      +20     
- Misses      17997    18013      +16     
+ Partials     5913     5880      -33     

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