Add dump-mir=<val> options

[spastorino]

rustc +nightly -Zdump-mir=<val> -Zmir-opt-level=3

-Z                              dump-mir=val -- dump MIR state to file.                                 
    `val` is used to select which passes and functions to dump. For example:                                        
    `all` matches all passes and functions,                                                                           
    `foo` matches all passes for functions whose name contains 'foo',                                                    
    `foo & ConstProp` only the 'ConstProp' pass for function names containing 'foo',   
    `foo | bar` all passes for function names containing 'foo' or 'bar'.