tcg: Optimize fence instructions

tcg: Optimize fence instructions

This commit optimizes fence instructions.  Two optimizations are
currently implemented: (1) unnecessary duplicate fence instructions,
and (2) merging weaker fences into a stronger fence.

[rth: Merge tcg_optimize_mb back into tcg_optimize, so that we only
loop over the opcode stream once.  Merge "unrelated" weaker barriers
into one stronger barrier.]

Signed-off-by: Pranith Kumar <[email protected]>
Message-Id: <20160823134825[email protected]>
Signed-off-by: Richard Henderson <[email protected]>