[BugFix] Skip DecoupleTypeCast on Evaluate roots to keep cp.async operands in their address spaces (#2869)

* [BugFix] Skip DecoupleTypeCast on Evaluate roots to keep cp.async operands in their address spaces

* [Test] Assert cp_async_gs lines exist before checking their operands

  The codegen regression test filtered kernel source with
  , so a source with no global->shared calls
  left the operand assertion with nothing to inspect and the test passed
  vacuously. The preceding  did not catch
  that: cp_async_commit/cp_async_wait come from the pipeline's sync
  skeleton, a separate emit site, and match that substring on their own.

  Collect the cp_async_gs lines, assert the collection is non-empty, then
  check each one for _local_cast operands.