Fix MachineInstr::findRegisterUseOperandIdx subreg checks

The function only checks that instruction reads a super-register
containing requested physical register. In case if a sub-register
if being read that is also a use of a super-reg, so added the check.
In particular MI->readsRegister() is broken because of the missing
check. The resulting check is essentially regsOverlap().

Differential Revision: https://reviews.llvm.org/D54128

llvm-svn: 346686