JIT: fix BasicBlock::isEmpty()

[AndyAyersMS]

The detection of blocks with only PHI assignments was broken by #50806.
Fix by using existing helper to find the first non-PHI assignment.

Closes #51326.

[AndyAyersMS]

@BruceForstall PTAL
cc @dotnet/jit-contrib

Will kick off jit stress for this PR shortly.

[BruceForstall]

LGTM. Thanks!

[AndyAyersMS]

One asm diff via SPMI (RA making different choices), a few text diffs (likely block weight shifts).

[AndyAyersMS]

Arm32 tests passed, but CI failed to report success:

=== TEST EXECUTION SUMMARY ===
   JIT.Regression.XUnitWrapper  Total: 152, Errors: 0, Failed: 0, Skipped: 0, Time: 26.731s
...
  WARNING: Retrying (Retry(total=1, connect=None, read=None, redirect=None, status=None)) after connection broken by 'NewConnectionError('<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0xeaa1ebf0>: Failed to establish a new connection: [Errno -3] Temporary failure in name resolution',)': /simple/azure-devops/
...
[EXECUTION TIMED OUT]
Exit Code:-3Executor timed out after 1800 seconds and was killed
['JIT.Regression.CLR-x86-JIT.V1-M12-M13' WORK ITEM LOG TRUNCATED: Command timed out, and was killed]

[AndyAyersMS]

Jitstress ongoing, one failure so far, in coreroot_determinism.sh.

[AndyAyersMS]

Also seeing the burgers failure #51293.

[BruceForstall]

Failures are known