[RISCV] Remove redundant czero in multi-word comparisons (#180485)

When comparing multi-word integers with Zicond, we generate:
  (or (czero_eqz (lo1 < lo2), (hi1 == hi2)),
      (czero_nez (hi1 < hi2), (hi1 == hi2)))

The czero_nez is redundant because when hi1 == hi2 is true, hi1 < hi2 is
already 0. This patch adds a DAG combine to recognize:
  czero_nez (setcc X, Y, CC), (setcc X, Y, eq) -> (setcc X, Y, CC)
when CC is a strict inequality (lt, gt, ult, ugt).

This saves one instruction in 128-bit comparisons on RV64 with Zicond.

Note the czero_nez becomes a czero.eqz in the final assembly because the
seteq is replaced by an xor that produces 0 when the values are equal.

Part of #179584

Assisted-by: claude

0 files changed, 0 insertions, 0 deletions