Simplify logic in Diagonal-Tridiagonal multiplication

[jishnub]

This improves performance in small ::Diagonal * ::Tridiagonal multiplications:

julia> D = Diagonal(1:3); T = Tridiagonal(1:2, 1:3, 1:2); C = similar(T);

julia> @btime mul!($C, $D, $T);
  43.435 ns (0 allocations: 0 bytes) # master
  14.725 ns (0 allocations: 0 bytes) # this PR

This also improves TTFX

julia> @time mul!(C, D, T);
  0.192970 seconds (451.55 k allocations: 22.211 MiB, 99.99% compilation time) # master
  0.129910 seconds (273.38 k allocations: 13.351 MiB, 99.98% compilation time) # this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.74%. Comparing base (5165fd3) to head (993462b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1338      +/-   ##
==========================================
- Coverage   93.74%   93.74%   -0.01%     
==========================================
  Files          34       34              
  Lines       15775    15766       -9     
==========================================
- Hits        14789    14780       -9     
  Misses        986      986              

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