[Mips] Simplify isShiftedUIntAtAnyPosition (NFC)
isShiftedUIntAtAnyPosition never gets zero as the argument because the
caller processes ImmValue satisfying isInt<16>(ImmValue), which
includes zero, long before it calls isShiftedUIntAtAnyPosition.
Given that the argument is always nonzero, findFirstSet is identical
to llvm::countr_zero. Also, x == x >> BitNum << BitNum is always
true, so we are left with:
isUInt<N>(x >> llvm::countr_zero(x))
Just in case the caller changes its behavior and starts passing zero
to us, we can protect the shift from undefined behavior "x << 64" by
adding "x &&".