Fix GEMM dispatch for complex-real matmul

[jishnub]

This should fix #1519. The issue on master is that we have specialized dispatches for arrays of the same eltype, and the complex-real matmul ends up in generic_matmatmul!. This adds an extra method to ensure that the complex-real case also reaches BLAS.

julia> A = ones(ComplexF64, 400, 400); B = ones(size(A)); C = similar(A);

julia> @btime mul!($C, $A, $B);
  57.043 ms (0 allocations: 0 bytes) # master
  1.608 ms (0 allocations: 0 bytes) # this PR

[dkarrasch]

Is this redirecting to the "reinterpret-trick"? Did that get lost over the more recent changes?

[adienes]

just for posterity if it happens to be useful, I had just bisected this to JuliaLang/julia#55002

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.03%. Comparing base (f41bf65) to head (cbdb717).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1520      +/-   ##
==========================================
+ Coverage   93.94%   94.03%   +0.09%     
==========================================
  Files          34       34              
  Lines       15984    15980       -4     
==========================================
+ Hits        15016    15027      +11     
+ Misses        968      953      -15     

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[jishnub]

@dkarrasch Exactly, this had regressed from the previous behavior.

[jishnub]

@adienes Thanks for the bisect! It appears that the fix required is a bit different. The method already existed, but the BLAS flag format had changed between releases and hadn't been updated.

[jishnub]

I don't think this method is ever taken. BlasReal matrices would always use GEMM as the flag, and gemm_wrapper! may delegate to _generic_matmatmul! if necessary.