Support looking up registers

[werat]

Add support for looking up registers via $rax syntax and plain rax (as a variable). The first is supported by LLDB's expression evaluator and the second is used by the Visual Studio C++ debugger.

When looking up as a variable, any other identifier (local or global variable, member field, etc) will shadow the register.

[github-actions[bot]]

Benchmark comparison for 153b75da (base) vs 12f26e34 (PR)

Benchmark                             Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------
BM/AddTwoNumbers                   +0.0013         +0.0012        118166        118322        118149        118289
BM/ArrayDereference                -0.0043         -0.0038        121993        121470        121921        121457
BM/ArraySubscript                  -0.0068         -0.0068        158764        157681        158746        157665
BM/TypeCasting                     -0.0109         -0.0112        356164        352277        356055        352073
BM/ParseInteger                    -0.0050         -0.0050        107604        107068        107594        107052

[github-actions[bot]]

Benchmark comparison for 153b75da (base) vs 0befc910 (PR)

Benchmark                             Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------
BM/AddTwoNumbers                   +0.0206         +0.0195        113374        115707        113302        115509
BM/ArrayDereference                +0.0249         +0.0251        120259        123251        120204        123224
BM/ArraySubscript                  -0.0671         -0.0670        164472        153438        164417        153403
BM/TypeCasting                     +0.0343         +0.0341        349810        361798        349755        361666
BM/ParseInteger                    +0.0862         +0.0861        105060        114114        105044        114094

[github-actions[bot]]

Benchmark comparison for 153b75da (base) vs c9d0c393 (PR)

Benchmark                             Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------
BM/AddTwoNumbers                   +0.1698         +0.1698        102139        119478        102113        119455
BM/ArrayDereference                -0.1321         -0.1319        124936        108427        124886        108408
BM/ArraySubscript                  +0.1422         +0.1419        138089        157720        138069        157664
BM/TypeCasting                     -0.0024         -0.0024        314977        314234        314940        314199
BM/ParseInteger                    -0.1227         -0.1225        107903         94668        107873         94658