[MIPS]: Rework atomic max/min expand for subword (#89575)

The current code is so buggy: it can work for few cases. The problems
include:
1. ll/sc works on a whole word, while other parts other than we rmw are
dropped.
  2. The oprands are not well zero-extended for unsigned ops.
3. It doesn't work for big-endian, as the postion of subword differs
with little endian.

And in fact, we can set the return value correct in ll/sc scope, so we
can skip the sinkMBB.