julia.loopinfo_marker can be attaced to wrong loop.

[vchuravy]

Noticed by @lcw in JuliaGPU/KernelAbstractions.jl#411

Using the snippet in the PR above we see:

➜  julia JULIA_LLVM_ARGS="-debug-only=lower_simd_loop" ./julia --project=~/src/KernelAbstractions ~/src/KernelAbstractions/codegen.jl
LSL: loopinfo marker found
LSL: loop header: 
L2:                                               ; preds = %L2, %top
  %value_phi = phi i64 [ 1, %top ], [ %7, %L2 ]
  %2 = sitofp i64 %value_phi to double, !dbg !30
  %3 = add i64 %value_phi, -1, !dbg !37
  %4 = bitcast {} addrspace(10)* %"a::Array" to double addrspace(13)* addrspace(10)*, !dbg !37
  %5 = addrspacecast double addrspace(13)* addrspace(10)* %4 to double addrspace(13)* addrspace(11)*, !dbg !37
  %.data10 = load double addrspace(13)*, double addrspace(13)* addrspace(11)* %5, align 8, !dbg !37, !tbaa !46, !alias.scope !49, !noalias !52, !nonnull !11
  %6 = getelementptr inbounds double, double addrspace(13)* %.data10, i64 %3, !dbg !37
  store double %2, double addrspace(13)* %6, align 8, !dbg !37, !tbaa !57, !alias.scope !60, !noalias !61
  call void @julia.loopinfo_marker(), !dbg !62, !julia.loopinfo !63
  %.not.not = icmp eq i64 %value_phi, 1000, !dbg !65
  %7 = add i64 %value_phi, 1, !dbg !68
  br i1 %.not.not, label %L20, label %L2, !dbg !71

LSL: has julia.loopinfo metadata with 1 operands
LSL: simd: 0 ivdep: 0

The Loop is not in in idiom form (e.g loop header and latch are collapsed into one BB) and simplifycfg eliminated the inner loop.

This issue could also occur with @simd and thus cause miscompilations.