[Loopinfo] recursive conversion to metadata

[vchuravy]

Follow-up on #31095, instead of pattern-matching against a limited set of patterns allow for a full description of a metadata tree.
Run into this because LLVM required a MDNode around the MDString for llvm.loop.unroll.full. This PR allows us to write that as: Expr(:loopinfo, ("llvm.loop.unroll.full",))

Right now one can work around that particular one by using Expr(:loopinfo, ("llvm.loop.unroll.full", true)), the second element of the tuple gets ignored.
Also a good opportunity to add more tests.

I would like to eventually enable,

function f(::Val{N}) where N
    @unroll N for i in ...
        ...
     end
end

e.g. using compile-time constants (and not parse time constants) as part of the build expression, but that requires a bigger rework of the node, to allow SSAValues.

cc: @lcw

[vchuravy]

Also addresses #31095 (comment)

[vchuravy]

The last commit is in response to the failure on Linux:

julia --startup-file=no /home/travis/build/JuliaLang/julia/test/llvmpasses/loopinfo.jl /home/travis/build/JuliaLang/julia/test/llvmpasses/Output/loopinfo.jl.tmp | llvm-link -S /home/travis/build/JuliaLang/julia/test/llvmpasses/Output/loopinfo.jl.tmp/* | FileCheck /home/travis/build/JuliaLang/julia/test/llvmpasses/loopinfo.jl
julia --startup-file=no /home/travis/build/JuliaLang/julia/test/llvmpasses/loopinfo.jl /home/travis/build/JuliaLang/julia/test/llvmpasses/Output/loopinfo.jl.tmp | llvm-link -S /home/travis/build/JuliaLang/julia/test/llvmpasses/Output/loopinfo.jl.tmp/* | opt -load libjulia.so -LowerSIMDLoop -S - | FileCheck /home/travis/build/JuliaLang/julia/test/llvmpasses/loopinfo.jl -check-prefix=LOWER
--
Exit Code: 2
Command Output (stderr):
--
llvm-link: /home/travis/build/JuliaLang/julia/test/llvmpasses/Output/loopinfo.jl.tmp/*: error: Could not open input file: No such file or directory
llvm-link: error loading file '/home/travis/build/JuliaLang/julia/test/llvmpasses/Output/loopinfo.jl.tmp/*'
FileCheck error: '-' is empty.
FileCheck command line:  FileCheck /home/travis/build/JuliaLang/julia/test/llvmpasses/loopinfo.jl

[vchuravy]

@staticfloat buildbot fail on upload to AWS.

[vchuravy]

Ok this is done from my side, the travis MacOSX error is a time-out. Absent objections I will merge this tomorrow.