Refactor Expr(:simdloop) to Expr(:loopinfo, ...)

[vchuravy]

• Expr(:loopinfo, Symbol("julia.simdloop")) replaces Expr(:simdloop, false)
• Expr(:loopinfo, Symbol("julia.simdloop"), Symbol("julia.ivdep")) replaces Expr(:simdloop, true)

While currently not directly exposed, users can now pass other loopinfo metadata to LLVM.
Codegen attaches the loop metadata to the call to the marker and the LowerSIMD pass consumes that metadata and
attaches it to the loop.

@eval function unroll(X)
           acc = 0.0
           @inbounds for i in 1:length(X)
               acc += X[i]
               $(Expr(:loopinfo, (Symbol("llvm.loop.unroll.count"), 4),
                                 (Symbol("llvm.loop.vectorize.enable"), false)))
           end
           return acc
       end

I needed this for #31086, this also removes unneeded and untested machinery from llvm-simdloop.cpp.

[vchuravy]

@nanosoldier runbenchmarks(ALL, vs = ":master")

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @ararslan

[vchuravy]

Nanosoldier seems fine (if this would lead to failed vectorisation I would expect slow downs on the oder of 4x)

@Keno good to merge or so you have comments about the current implementation?

[Keno]

Fine with me.

[vtjnash]

/Users/jameson/julia/src/codegen.cpp:4109:24: warning: variable 'MD' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized]
            } else if (jl_is_tuple(arg)) {
                       ^~~~~~~~~~~~~~~~
./julia.h:959:30: note: expanded from macro 'jl_is_tuple'
#define jl_is_tuple(v)       (((jl_datatype_t*)jl_typeof(v))->name == jl_tuple_typename)
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/jameson/julia/src/codegen.cpp:4127:17: note: uninitialized use occurs here
            if (MD)
                ^~
/Users/jameson/julia/src/codegen.cpp:4109:20: note: remove the 'if' if its condition is always true
            } else if (jl_is_tuple(arg)) {
                   ^~~~~~~~~~~~~~~~~~~~~~
/Users/jameson/julia/src/codegen.cpp:4104:25: note: initialize the variable 'MD' to silence this warning
            Metadata *MD;
                        ^
                         = nullptr
1 warning generated.