[SDAG] try to convert bit set/clear to signbit test when trunc is free

(X & Pow2MaskC) == 0 --> (trunc X) >= 0
(X & Pow2MaskC) != 0 --> (trunc X) <  0

This was noted as a regression in the post-commit feedback for D112634
(where we canonicalized IR differently).

For x86, this saves a few instruction bytes. AArch64 seems neutral.

Differential Revision: https://reviews.llvm.org/D139363