Allows easier structural comparison of Cmm code.

Before:
```
       cxCH: // global
           _suEU::P64 = R1;
           if ((Sp + -16) < SpLim) (likely: False) goto cxCI; else goto
cxCJ;
```

After
```
       _lbl_: // global
           __locVar_::P64 = R1;
           if ((Sp + -16) < SpLim) (likely: False) goto cxBf; else goto
cxBg;
```

Test Plan: Looking at dumps, ci

Reviewers: bgamari, simonmar

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4786