[AArch64/ARM] Fix two compiler warnings in InstructionSelector, NFCI
- GCC complains that KnownValid is set but not used.
- In ARMInstructionSelector::selectGlobal() the code is mixing "enumeral and non-enumeral type in conditional expression". Solve this by casting to unsigned which is the final type anyway.
Differential Revision: https://reviews.llvm.org/D58834