[patch] Cleanup tree-switch-conversion a bit

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

> My goal for GCC 4.8 is to do just that: Move switch expansion to
> GIMPLE and add value profiling for switch expressions.

And the idea is to put all that code in tree-switch-conversion.c. But
there are a few clean-ups I wish to do on that code before that.
First, there is a global pass info structure that contains useful data
for all forms of GIMPLE_SWITCH lowering. I've "un-globalized" that
data with the attached patch. While there, I made the dump messages
uniform.

Bootstrapped and tested on powerpc-unknown-linux-gnu. OK?

Ciao!
Steven

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