Bug 913282: IsFloat32Representable function in mfbt; r=Waldo

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extra : rebase_source : 0d4d6f02b880c37d29e3e41e9d105365d9f941b0
Bug 913282: More Float32 operators: UnsignedToFloat32; p=dougc,bbouvier, r=jonco,h4writer

dougc for the ARM parts, bbouvier for the rest
Bug 913282: Tests
Bug 918163: Tests