X86InstrInfo: No need for liveness analysis in classifyLEAReg()

classifyLEAReg() deals with switching operands from 32bit to 64bit in
order to use a LEA64_32 instruction (for three address code goodness).
It currently performs a liveness analysis to determine the kill/undef
flag for the newly added operand. This should not be necessary:

- If the previous operand had a kill flag, then the 32bit part of the
  register gets killed, this will kill the super register as well.
- If the previous operand had an undef flag then we didn't care what
  value we read, just use the same flag on the new operand.
  (No matter what an operand with an undef flag won't affect liveness)

This makes the code independent of the presence of kill flags because it
avoids a call to MachineBasicBlock::computeRegisterLiveness().

Differential Revision: http://reviews.llvm.org/D22283

llvm-svn: 276222