Explicit loop in _iszero for strided views

[jishnub]

This reduces TTFX

julia> using LinearAlgebra

julia> Z = zeros(4,4);

julia> @time istril(Z);
  0.152632 seconds (572.99 k allocations: 27.476 MiB, 99.98% compilation time) # master
  0.091427 seconds (177.39 k allocations: 8.688 MiB, 99.97% compilation time) # this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.00%. Comparing base (925acef) to head (41fb953).
Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1264      +/-   ##
==========================================
- Coverage   92.03%   92.00%   -0.04%     
==========================================
  Files          34       34              
  Lines       15475    15479       +4     
==========================================
- Hits        14243    14242       -1     
- Misses       1232     1237       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.