Add support for setting and querying fast math flags.

[maleadt]

As requested by @chriselrod

Backport llvm/llvm-project#75123 to LLVMExtra, and add a Julian API:

• fast_math(::Instruction) returning a named tuple
• fast_math!(::Instruction; flags...) specifying flags as kwargs as kwarg

I contemplated using symbols instead (fast_math!(inst, :contract)), but with kwargs its easier to pass multiple flags.

[maleadt]

Demo:

@dispose ctx=Context() begin
    # set-up
    mod = LLVM.Module("my_module")

    param_types = [LLVM.FloatType()]
    ret_type = LLVM.FloatType()
    fun_type = LLVM.FunctionType(ret_type, param_types)
    fun = LLVM.Function(mod, "add_sub", fun_type)

    # generate IR
    @dispose builder=IRBuilder() begin
        entry = BasicBlock(fun, "entry")
        position!(builder, entry)

        # add and substract 42
        a = fadd!(builder, parameters(fun)[1], LLVM.ConstantFP(Float32(42.)), "a")
        fast_math!(a; all=true)
        b = fsub!(builder, a, LLVM.ConstantFP(Float32(42.)), "b")
        fast_math!(b; all=true)

        ret!(builder, b)
    end
    verify(mod)
    display(mod)

    println("#"^92)
    println()

    # optimize IR
    host_triple = triple()
    host_t = Target(triple=host_triple)
    @dispose tm=TargetMachine(host_t, host_triple) pb=PassBuilder(tm) begin
        NewPMModulePassManager(pb) do mpm
            parse!(pb, mpm, "default<O3>")
            run!(mpm, mod, tm)
        end
    end
    verify(mod)
    display(mod)
end

; ModuleID = 'my_module'
source_filename = "my_module"

define float @add_sub(float %0) {
entry:
  %a = fadd fast float %0, 4.200000e+01
  %b = fsub fast float %a, 4.200000e+01
  ret float %b
}

############################################################################################

; ModuleID = 'my_module'
source_filename = "my_module"

; Function Attrs: mustprogress nofree norecurse nosync nounwind readnone willreturn
define float @add_sub(float returned %0) local_unnamed_addr #0 {
entry:
  ret float %0
}

attributes #0 = { mustprogress nofree norecurse nosync nounwind readnone willreturn }