Don't copy regs that are only used in the entry block into a vreg. This

changes the code generated for:

short %test(short %A) {
  %B = xor short %A, -32768
  ret short %B
}

to:

_test:
        xori r2, r3, 32768
        xoris r2, r2, 65535
        extsh r3, r2
        blr

instead of:

_test:
        rlwinm r2, r3, 0, 16, 31
        xori r2, r3, 32768
        xoris r2, r2, 65535
        extsh r3, r2
        blr

llvm-svn: 23109