Add identity match mir pass

[JulianKnodt]

Adds in a mir-optimization pass for optimizing over

match some_enum {
  Var1(x) => Var1(x),
  Var2(y) => Var2(y),
}

While browsing through old issues I found #38349, and thought it would be simple enough to write a mir optimization pass for it, even though the LLVM looks to be fairly optimized.

[rust-highfive]

r? @eddyb

(rust_highfive has picked a reviewer for you, use r? to override)

[oli-obk]

r? @oli-obk

[oli-obk]

it doesn't look like your optimization actually gets applied

[JulianKnodt]

ah sorry, I left this up for a while because I was not sure if it was a good idea, but I think I'll just close it because it's likely to slow down the parsing more and can probably just be handled when the MIR is created.