spurious crash in 2.11.x in genLoadIdent

[adriaanm]

symbol value c#16812 does not exist in Macro.impl, which contains locals value a#16810, value b#16811

We're trying to diagnose this (e.g. via scala/scala#4397), but no idea what's going on.

[retronym]

Also tracked as https://issues.scala-lang.org/browse/SI-9264