[DAGCombiner] Fold subtraction if above threshold to `umin` (#134235)

Folds patterns such as:

    unsigned foo(unsigned x, unsigned y) {
      return x >= y ? x - y : x;
    }

Before, on RISC-V:

    sltu    a2, a0, a1
    addi    a2, a2, -1
    and     a1, a1, a2
    subw    a0, a0, a1

Or, with Zicond:

    sltu    a2, a0, a1
    czero.nez a1, a1, a2
    subw    a0, a0, a1

After, with Zbb:

    subw    a1, a0, a1
    minu    a0, a0, a1

Only applies to unsigned comparisons.
If `x >= y` then `x - y` is less than or equal `x`.
Otherwise, `x - y` wraps and is greater than `x`.

0 files changed, 0 insertions, 0 deletions