[patch] Cleanup tree-switch-conversion a bit

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

Hello,

This is another step towards moving GIMPLE_SWITCH expansion to an
earlier point in the pipeline.

With the attached patch, some of the logic from stmt.c:add_case_node()
is moved to gimplify.c:gimplify_switch_expr(). This includes:

* Code to drop case labels that are out of range for the switch index
expression. (Actually, I suspect this code hasn't worked properly
since gimplification was introduced, because the switch index
expression can be promoted by language specific gimplification, so
expand_case never actually sees the proper type with the current
implementation in stmt.c.)

* Code to fold_convert case label values to the right type. I've opted
to go for folding to the original type of the SWITCH_EXPR, rather than
to the post-gimplification switch index type.

* Code to canonicalize CASE_LABEL's subnodes, CASE_LOW and CASE_HIGH.
I've chosen to impose strict requirements that CASE_HIGH > CASE_LOW if
CASE_HIGH is non-zero. This is different from what add_case_node does,
but I think it makes sense to go for the minimal representation here:
The case labels in stmt.c never lived very long (only during expand)
but GIMPLE_SWITCH statements stay around for much of the compilation
process and can also be streamed out, etc.

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

Ciao!
Steven

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