Fallback path: annotate `for` loops with `@simd`?

Here's the fallback path for the basic matmul kernel:
```julia
$(Expr(:inbounds, true))
local var"#1#val" = for n = 1:size(A, 1), m = 1:size(B, 2)
#= REPL[2]:2 =#
Cₙₘ = zero(eltype(C))
#= REPL[2]:3 =#
for k = 1:size(A, 2)
#= REPL[2]:4 =#
Cₙₘ = Base.FastMath.add_fast(Cₙₘ, Base.FastMath.mul_fast(A[n, k], B[k, m]))
end
#= REPL[2]:6 =#
C[n, m] = Cₙₘ
end
$(Expr(:inbounds, :pop))
var"#1#val"
```

In the fallback path, `@inbounds` and `@fastmath` have both been applied.

Would it also make sense to annotate the `for` loop in the fallback path with `@simd`?

Start with the fallback path for the basic matmul kernel shown in the issue and compare its existing @inbounds and @fastmath annotations with the proposed @simd annotation. Determine the relevant fallback entry point and any associated tests before changing anything; the issue names no source file or test, so completion would require validating whether the loop annotation is appropriate.

Scritto dal modello di indicizzazione a partire dal testo della issue.