Remove LLVM instruction stats and other (obsolete) codegen stats.
Both `-Z count_llvm_insns` and `-Z codegen-stats` are removed, as (AFAIK) they have been of little use in the last few years, especially after the transition to MIR->LLVM codegen.
Other than for the LLVM instruction counts, `-Z codegen-stats` has long been obsoleted anyway.
r? @nagisa cc @rust-lang/compiler
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc guide.
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