re-align 2.10.x's pattern matcher with master's

The diff was mostly code cleanup, so most of that was propagated from master to 2.10.x.
The remaining diff is encapsulated in compatibility stubs (see below).
(There was also the on/off potential for the 2.10.x "new" pattern matcher.
 The old one is gone in 2.11, so no turning off new patmat, of course.)

The stubs:

```
protected final def dealiasWiden(tp: Type)   = tp.dealias                       // 2.11: dealiasWiden
protected final def mkTRUE                   = CODE.TRUE_typed                  // 2.11: CODE.TRUE
protected final def mkFALSE                  = CODE.FALSE_typed                 // 2.11: CODE.FALSE
protected final def hasStableSymbol(p: Tree) = p.hasSymbol && p.symbol.isStable // 2.11: p.hasSymbolField && p.symbol.isStable
protected final def devWarning(str: String)  = global.debugwarn(str)            // 2.11: omit
```

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