- Change MachineInstr::findRegisterDefOperandIdx so it can also look for defs

that are aliases of the specified register.
- Rename modifiesRegister to definesRegister since it's looking a def of the
specific register or one of its super-registers. It's not looking for def of a
sub-register or alias that could change the specified register.
- Added modifiesRegister to look for defs of aliases.

llvm-svn: 104377