[DAG] Attempt to fold bswap(shl(x,c)) -> zext(bswap(trunc(shl(x,c-bw/2))))

If the shl is at least half the bitwidth (i.e. the lower half of the bswap source is zero), then we can reduce the shift and perform the bswap at half the bitwidth and just zero extend.

Based off PR51391 + PR53867

Differential Revision: https://reviews.llvm.org/D120192