Improve LIR dumping (dotnet/coreclr#10140)

Improve LIR dumping (dotnet/coreclr#10140)

1. Use the LIR node dumper to display nodes to be generated by
codegen, since we're in LIR form at that point. Add a new
"prefix message" argument to allow "Generating: " to prefix all
such lines.
2. Fix off-by-one error in LIR dump due to `#ifdef` versus `#if`.
3. Remove extra trailing line for each LIR node. This interfered
with dotnet/coreclr#1. But I always thought it was unnecessarily verbose; I don't
believe there is any ambiguity without that extra space.
4. Add dTreeLIR()/cTreeLIR() functions for use in the debugger.

Commit migrated from https://github.com/dotnet/coreclr/commit/8d749d34e8f028580d923d8514e4926e16234551