Quick assembler fix

This CL re-instates the select pattern optimization disabled by
CL 374310, and fixes the underlying problem: improper handling of
the kPseudoBarrier LIR opcode.  The bug was introduced in the
recent assembler restructuring.  In short, LIR pseudo opcodes (which
have values < 0), should always have size 0 - and thus cause no
bits to be emitted during assembly.  In this case, bad logic caused
us to set the size of a kPseudoBarrier opcode via lookup through the
EncodingMap.

Because all pseudo ops are < 0, this meant we did an array underflow
load, picking up whatever garbage was located before the EncodingMap.
This explains why this error showed up recently - we'd previuosly just
gotten a lucky layout.

This CL corrects the faulty logic, and adds DCHECKs to uses of
the EncodingMap to ensure that we don't try to access w/ a
pseudo op.  Additionally, the existing is_pseudo_op() macro is
replaced with IsPseudoLirOp(), named similar to the existing
IsPseudoMirOp().

Change-Id: I46761a0275a923d85b545664cadf052e1ab120dc