Turn stuff like:
Turn stuff like:

        icmp slt i32 %X, 0              ; <i1>:0 [#uses=1]
        sext i1 %0 to i32               ; <i32>:1 [#uses=1]

into:

        %X.lobit = ashr i32 %X, 31              ; <i32> [#uses=1]

This implements InstCombine/icmp.ll:test[34]


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@35891 91177308-0d34-0410-b5e6-96231b3b80d8

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