overzealous assertion in BCodeBodyBuilder
BCodeOpt building block: PushPopCollapser

Back-propagate POP and POP2 instructions to the instructions
producing the values to drop; all while preserving
and side-effects of those instructions.

Details in the documentation for
scala.tools.nsc.backend.bcode.PushPopCollapser
Level 1 intra-method optimiz: PushPopCollapser