Switch case-ranges, new approach (PR1255).

[Stepan Dyatkovskiy]

Hello, I propose new approach to switch case-ranges
implementation (PR1255).

Don't modify SwitchInst, but implement new instruction.

In this case:
1. We don't affect current functionality and tests.
2. We keep backward compatibility in natural way. If you need current
functionality (without case ranges) just use 'switch'.

Here I have attached set of patches:

-- New instruction itself. It is called 'SwitchRInst'. Implementation
    is ConstantRange based. Internally switch is represented as just an
    array of integers. Though i-th and i+1 integer represents a range,
    and i-th integer is linked with this-range successor. This is a
    result of implementation discussions in IRC and ML.
    Patch name: switchr-2013-05-07-instruction.patch

-- Parser/AsmWriter patch. After this patch IR parser will
    recognize new instruction and its syntax ('switchr' keyword will
    be added).
    Patch contains unit-tests.
    Patch name: switchr-2013-05-07-parser-writer.patch
    Requires: switchr-2013-05-07-instruction.patch

-- Bitcode patch. Patch adds new instruction support
    for .bc files with.
    Patch contains unit-tests.
    Patch name: switchr-2013-05-07-bitcode.patch
    Requires: switchr-2013-05-07-instruction.patch
             [switchr-2013-05-07-parser-writer.patch] (for unit-tests)

-- "Essential passes" patch. This patch finally enables 'switchr':
    it becomes absultely legal on LLVM level. Patch updates next passes:
    * SelectionDAGBuilder: lowers 'switchr';
    * IRBuilder: "Create" method;
    * SCCP: just a stub, that marks all 'switchr' successors
    as reachable.
    Patch contains unit-tests.
    Patch name: switchr-2013-05-07-essential-passes.patch
    Requires: switchr-2013-05-07-instruction.patch
             [switchr-2013-05-07-parser-writer.patch] (for unit-tests)

-- Switch-to-SwitchR pass patch. Adds function pass that replaces
    'switch' instructions with 'switchr'.
    Patch name: switchr-2013-05-07-switch-to-switchr.patch

-- Clang patch. Adds '-fforce-switchr' option: with this flag clang
    will use 'switchr' instructions instead of 'switch'. Actually, it
    inserts SwitchToSwitchR patch.
    Patch contains unit-tests.
    Patch name: switchr-clang-force-flag-2013-05-07.patch
    Requires: switchr-2013-05-07-instruction.patch
              switchr-2013-05-07-switch-to-switchr.patch
             [switchr-2013-05-07-parser-writer.patch] (for unit-tests)

Also I've attached summary LLVM patch (all-in-one).

-Stepan.
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