[AArch64/ARM] Fix two compiler warnings in InstructionSelector, NFCI

1) GCC complains that KnownValid is set but not used.
2) 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

llvm-svn: 355304