[scan-build] Fix reported logic error bugs

[Apelete Seketeli via llvm-commits]

Hello,

Here are a few changes that try to address some logic error bugs
reported by running scan-build over the LLVM code base.

I have no background in compiler technology and I may have been trying
to fix false positives; here is a quick overview of what scan-build
reported, let me know if the fixes are relevant:

Bug group: Logic error
Bug Type: Called C++ object pointer is null
Files: lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
    
Bug group: Logic error
Bug Type: Called C++ object pointer is null
Files: lib/CodeGen/AsmPrinter/DwarfUnit.cpp
    
Bug group: Logic error
Bug Type: Branch condition evaluates to a garbage value
Files: lib/Target/Hexagon/HexagonInstrInfo.cpp

Bug group: Logic error
Bug Type: Assigned value is garbage or undefined
Files: lib/Target/TargetRecip.cpp

Cheers.
-- 
        Apelete
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