[PowerPC] Remove old README.txt entry

We no longer generate horrible code for the stated function:

void f(signed char *a, _Bool b, _Bool c) {
  signed char t = 0;
  if (b)  t = *a;
  if (c)  *a = t;
}

for which we now generate:

.L.f:
        andi. 5, 5, 1
        cmpldi 1, 4, 0
        li 5, 0
        beq 1, .LBB0_2
        lbz 5, 0(3)
.LBB0_2:                                # %if.end
        bclr 4, 1, 0
        stb 5, 0(3)
        blr

so we don't need the README.txt entry.

llvm-svn: 225217