Auto merge of #34189 - eddyb:mir-trans-imm, r=nagisa

Auto merge of #34189 - eddyb:mir-trans-imm, r=nagisa

trans: generalize immediate temporaries to all MIR locals.

Added `Mir::local_index` which gives you an unified index for `Arg`, `Var`, `Temp` and `ReturnPointer`.
Also available is `Mir::count_locals` which returns the total number of the above locals.
This simplifies a lot of the code which can treat all of the local lvalues in the same manner.
If we had `-> impl Iterator`, I could have added a bunch of useful `Ty` or `Lvalue` iterators for all locals.
We could of course manually write such iterators as they are needed.

The only place which currently takes advantage of unified locals is trans' alloca elision.
Currently it's not as good as it could be, due to our usage of `llvm.dbg.declare` in debug mode.
But passing some arguments and variables as immediates has some effect on release-mode `libsyntax`:

Old trans:
```
time: 11.500; rss: 710MB        translation
time: 0.002; rss: 710MB assert dep graph
time: 0.000; rss: 710MB serialize dep graph
  time: 4.410; rss: 628MB       llvm function passes [0]
  time: 84.485; rss: 633MB      llvm module passes [0]
  time: 23.898; rss: 634MB      codegen passes [0]
  time: 0.002; rss: 634MB       codegen passes [0]
time: 113.408; rss: 634MB       LLVM passes
```
`-Z orbit`, previously:
```
time: 12.588; rss: 723MB        translation
time: 0.002; rss: 723MB assert dep graph
time: 0.000; rss: 723MB serialize dep graph
  time: 4.597; rss: 642MB       llvm function passes [0]
  time: 77.347; rss: 646MB      llvm module passes [0]
  time: 24.703; rss: 648MB      codegen passes [0]
  time: 0.002; rss: 615MB       codegen passes [0]
time: 107.233; rss: 615MB       LLVM passes
```
`-Z orbit`, after this PR:
```
time: 13.820; rss: 672MB        translation
time: 0.002; rss: 672MB assert dep graph
time: 0.000; rss: 672MB serialize dep graph
  time: 3.969; rss: 591MB       llvm function passes [0]
  time: 72.294; rss: 595MB      llvm module passes [0]
  time: 24.610; rss: 597MB      codegen passes [0]
  time: 0.002; rss: 597MB       codegen passes [0]
time: 101.439; rss: 597MB       LLVM passes
```